Support Azure Machine Learning managed identity

[joshfree]

Library name

Azure.Identity

Please describe the feature.

The Azure ML managed identity API is no longer the same as the App Service 2017-09-01 API, so we need to add explicit support. The main differences vs. App Service are that expires_on is an int and token requests must specify a clientid parameter, even for system assigned identities. The platform provides a default client ID in environment variable DEFAULT_IDENTITY_CLIENT_ID. See Azure/azure-sdk-for-go#21851 for an example implementation and here's a test recording.