Feature: Add On-Behalf-Of (OBO) Auth Flow for the Microsoft Graph Team

[joshfree]

The OAuth 2.0 On-Behalf-Of flow (OBO) serves the use case where an application invokes a service/web API, which in turn needs to call another service/web API. The idea is to propagate the delegated user identity and permissions through the request chain. For the middle-tier service to make authenticated requests to the downstream service, it needs to secure an access token from the Microsoft identity platform, on behalf of the user.

Related Links:
Prototype: jongio/azidext#41
Documenation: https://docs.microsoft.com/en-us/azure/active-directory/develop/v2-oauth2-on-behalf-of-flow
.NET Design: https://gist.github.com/christothes/15b9903f7da0954aee166166a8fa1919
MS Graph Team's tracking issue: microsoftgraph/msgraph-sdk-java-core#132