Support Federated Identity Credentials (MSI + App) for App Registration authentication in the OBO flow

[naizerjohn-ms]

Hello,
Currently for the Application Registration (On-Behalf-Of) there is only support for use with client secrets. A similar issue #772 requests the feature for use of client certificates.

We are requesting the support for Federated Identity Credentials, specifically MSI credentials, for use in the App Registration. All that would need to be provided would be the Managed Identity Client ID as a field. This Managed Identity Client ID would then be used to retrieve a JWT token and passed as a client assertion in the POST request when requesting access tokens. The Golang code for this is provided here.

We will be working on a PR for this solution and wanted to create a discussion post/issue for visibility as well.