[Identity] Samples aligned with other languages (v1 compatible) (#15031)

In this PR we're aligning the samples of Identity with other languages. The samples I'm adding in this PR are:

- Rolling certificates with ClientCertificateCredential ([.Net](https://github.com/Azure/azure-sdk-for-net/blob/master/sdk/identity/Azure.Identity/samples/ClientCertificateCredentialSamples.md#rolling-certificates)).
- ClientCertificateCredential with a Key Vault Certificate ([Python](Azure/azure-sdk-for-python#18109)) (decided not to use `@azure/keyvault-certificates` because of the possible issues converting certificates from one format to another in Node).
- Custom credentials ([.Net](https://github.com/Azure/azure-sdk-for-net/blob/master/sdk/identity/Azure.Identity/samples/DefiningCustomCredentialTypes.md#defining-custom-credential-types)).
- Using MSAL directly ([.Net](https://github.com/Azure/azure-sdk-for-net/blob/master/sdk/identity/Azure.Identity/samples/DefiningCustomCredentialTypes.md#authenticating-with-msal-directly)).
- On Behalf Of Flow ([.Net](https://github.com/Azure/azure-sdk-for-net/blob/master/sdk/identity/Azure.Identity/samples/DefiningCustomCredentialTypes.md#authenticating-with-the-on-behalf-of-flow)).

New samples related to the v2-specific features will be added after the Identity package is separated.

Related to #14435

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I’ve decided to use this opportunity to:

Fix #15324

This PR also implicitly:

Fixes #15573