Added support for DEFAULT_IDENTITY_CLIENT_ID environment variable in Machine Learning Managed Identity

[Robbie-Microsoft]

Fixes #5350

If the Machine Learning System Assigned Managed Identity is being used, the "clientid" request parameter will be set to the DEFAULT_IDENTITY_CLIENT_ID environment variable instead of being omitted in the network request.

[gladjohn]

looks good @Robbie-Microsoft, but there are few MI test failures, are they related?

[bgavrilMS]

Do not make breaking changes to AppService2017, instead just add support for this Default_Client_id stuff

[gladjohn]

Do not make breaking changes to AppService2017, instead just add support for this Default_Client_id stuff

a well explained bug on the changes we are making will be a good place to start with. The reason why I ask this is, It is not clear if Azure ML Source and Azure ML API source are same or are they different?

My understanding, is Azure ML uses App Service 2017. Does Azure ML API also use App Service 2017?

[chlowell]

My understanding, is Azure ML uses App Service 2017. Does Azure ML API also use App Service 2017?

Azure ML and App Service have always had different managed identity implementations. Azure ML imitated App Service 2017 when designing their managed identity API, however the implementations never behaved the same and Azure ML's API is now incompatible with App Service 2017 in that it requires clientid for all token requests.