Cloud Shell doesn't support user assigned identities

[chlowell]

Cloud Shell's token endpoint ignores unexpected parameters and always authenticates the signed in user. This means that when ManagedIdentityCredential requests a token for a user assigned identity, Cloud Shell returns a token for the signed in user instead. The credential should therefore throw an authentication failed exception when asked to get a token for a user assigned identity in Cloud Shell.

[KarishmaGhiya]

As decided, Throwing an exception will tie the client side behaviour to Service side impl details, if they change impl on service side in future, our logic of throwing exceptions can break. Instead, we will do some logging here to warn users on known targeted platforms and add some details to docs too.

[KarishmaGhiya]

Fixed by #20226