docs: add Azure Repos MI/SP docs (#1548)

Add documentation about Managed Identities and Service Principals for
Azure Repos; how to configure them and use them with GCM.

This will be useful for those that wish to adopt them as alternatives to
PATs or other automated auth systems.

[Rendered
version](https://github.com/git-ecosystem/git-credential-manager/blob/8b93d631e315ca0d2563f28ce582439e19c0ac46/docs/azrepos-misp.md)

docs/README.md

@@ -11,18 +11,19 @@ The following are links to GCM user support documentation:
 - [Credential stores][gcm-credstores]
 - [Host provider specification][gcm-host-provider]
 - [Azure Repos OAuth tokens][gcm-azure-tokens]
+- [Azure Managed Identities and Service Principals][gcm-misp]
 - [GitLab support][gcm-gitlab]
 - [Generic OAuth support][gcm-oauth]
 
 [gcm-azure-tokens]: azrepos-users-and-tokens.md
 [gcm-config]: configuration.md
 [gcm-credstores]: credstores.md
-[gcm-dev]: development.md
 [gcm-enterprise-config]: enterprise-config.md
 [gcm-env]: environment.md
 [gcm-faq]: faq.md
 [gcm-gitlab]: gitlab.md
 [gcm-host-provider]: hostprovider.md
+[gcm-misp]: azrepos-misp.md
 [gcm-net-config]: netconfig.md
 [gcm-oauth]: generic-oauth.md
 [gcm-usage]: usage.md

docs/azrepos-misp.md

@@ -0,0 +1,126 @@
+# Azure Managed Identities and Service Principals
+
+Git Credential Manager supports Managed Identities and Service Principals for
+authentication with Azure Repos. This document provides an overview of Managed
+Identities and Service Principals and how to use them with GCM.
+
+## Managed Identities
+
+Azure Managed Identities can be used to authenticate and authorize applications
+and services to access Azure resources. Managed Identities are a secure way to
+access Azure resources without needing to store credentials in code or
+configuration files.
+
+There are two types of Managed Identities:
+
+**System-assigned**
+
+System-assigned Managed Identities are tied to a specific Azure resource, such
+as a Virtual Machine or App Service. When a system-assigned Managed Identity is
+enabled, Azure creates an identity for the resource in the Azure AD tenant
+that's trusted by the subscription. The lifecycle of the identity is tied to the
+resource to which it's assigned.
+
+**User-assigned**
+
+User-assigned Managed Identities are created as standalone Azure resources and
+can be assigned to one or more Azure resources. This allows you to use the same
+Managed Identity across multiple resources.
+
+You can read more about Managed Identities in the
+[Azure documentation][az-mi].
+
+### How to configure Managed Identities
+
+In order to use a Managed Identity with GCM, you need to ensure that the Managed
+Identity has the necessary permissions to access the Azure Repos repository.
+
+You can read more about how to configure Managed Identities in the
+[Azure Repos documentation][azdo-misp].
+
+Once you have configured the Managed Identity, you can use it with GCM by simply
+setting one of the following environment variables or Git configuration options:
+
+**Git configuration:** [`credential.azreposManagedIdentity`][gcm-mi-config]
+
+**Environment variable:** [`GCM_AZREPOS_MANAGEDIDENTITY`][gcm-mi-env]
+
+Value|Description
+-|-
+`system`|System-Assigned Managed Identity
+`[guid]`|User-Assigned Managed Identity with the specified client ID
+`id://[guid]`|User-Assigned Managed Identity with the specified client ID
+`resource://[guid]`|User-Assigned Managed Identity for the associated resource
+
+You can obtain the `[guid]` from the Azure Portal or by using the Azure CLI
+to inspect the Managed Identity or resource.
+
+## Service Principals
+
+Azure Service Principals are used to authenticate and authorize applications and
+services to access Azure resources. Service Principals are similar in many ways
+to Managed Identities (in fact Service Principals are used under the hood to
+implement Managed Identities), but they have expliclty defined credentials that
+are not managed by Azure.
+
+There are a number of different ways to create and configure Service Principals,
+including using the Azure Portal or Azure CLI. You can read more about Service
+Principals in the [Azure documentation][az-sp].
+
+### How to configure Service Principals
+
+Much like with Managed Identities, to use a Service Principal with GCM you first
+need to ensure that the principal has the necessary permissions to access the
+Azure Repos repository.
+
+You can read more about how to configure Service Principals in the
+[Azure Repos documentation][azdo-misp].
+
+Once you have configured the Service Principal, you can use it with GCM by
+setting one of the following environment variables or Git configuration options:
+
+**Git configuration:** [`credential.azreposServicePrincipal`][gcm-sp-config]
+
+**Environment variable:** [`GCM_AZREPOS_SERVICE_PRINCIPAL`][gcm-sp-env]
+
+The format of the value for these options must be in the format:
+
+```text
+{tenantId}/{clientId}
+```
+
+Where `{tenantId}` is the Azure tenant ID and `{clientId}` is the client ID of
+the Service Principal. These values can be found in the Azure Portal or by using
+the Azure CLI to inspect the Service Principal.
+
+#### Authentication with Service Principals
+
+When using a Service Principal with GCM, you will also need to provide the
+client secret or certificate that is associated with the Service Principal.
+
+You can provide the client secret or certificate to GCM by setting one of the
+following environment variables or Git configuration options.
+
+Type|Git Configuration|Environment Variable
+-|-|-
+Client Secret|[`credential.azreposServicePrincipalSecret`][gcm-sp-secret-config]|[`GCM_AZREPOS_SP_SECRET`][gcm-sp-secret-env]
+Certificate|[`credential.azreposServicePrincipalCertificateThumbprint`][gcm-sp-cert-config]|[`GCM_AZREPOS_SP_CERT_THUMBPRINT`][gcm-sp-cert-env]
+
+The value for these options should be the client secret or the thumbrint of the
+certificate that is associated with the Service Principal.
+
+The certificate itself should be installed on the machine where GCM is running
+and should be installed in personal store the certificate store for either the
+current user or the local machine.
+
+[az-mi]: https://learn.microsoft.com/en-us/entra/identity/managed-identities-azure-resources/overview
+[az-sp]: https://learn.microsoft.com/en-us/entra/identity-platform/app-objects-and-service-principals?tabs=browser
+[azdo-misp]: https://learn.microsoft.com/en-us/azure/devops/integrate/get-started/authentication/service-principal-managed-identity?view=azure-devops
+[gcm-mi-config]: https://gh.io/gcm/config#credentialazreposmanagedidentity
+[gcm-mi-env]: https://gh.io/gcm/env#GCM_AZREPOS_MANAGEDIDENTITY
+[gcm-sp-config]: https://gh.io/gcm/config#credentialazreposserviceprincipal
+[gcm-sp-env]: https://gh.io/gcm/env#GCM_AZREPOS_SERVICE_PRINCIPAL
+[gcm-sp-secret-config]: https://gh.io/gcm/config#credentialazreposserviceprincipalsecret
+[gcm-sp-secret-env]: https://gh.io/gcm/env#GCM_AZREPOS_SP_SECRET
+[gcm-sp-cert-config]: https://gh.io/gcm/config#credentialazreposserviceprincipalcertificatethumbprint
+[gcm-sp-cert-env]: https://gh.io/gcm/env#GCM_AZREPOS_SP_CERT_THUMBPRINT