Access token retrieval in App service

[nitinprakash96]

App service provides two environment variables its instance for fetching access tokens:

1. IDENTITY_HEADER - authentication code representing the service on the current node
2. IDENTITY_ENDPOINT - local endpoint corresponding to service's managed identity

These two can then be used to fetch an access token. Note that this is particular to App service and will not work for standalone VM instances. At the moment withManagedIdentity will throw an error if you try fetching access token in an app service.

For more, see https://learn.microsoft.com/en-us/azure/service-fabric/how-to-managed-identity-service-fabric-app-code#acquiring-an-access-token-using-rest-api