Fallback to AZURE_CLIENT_ID env var if no `client_id` query param in token request

[aramase]

This is still up for debate so I didn't want to add this as part of this PR. I'll add a follow-up PR once we make a decision on this.

Originally posted by @aramase in https://github.com/Azure/aad-pod-managed-identity/pull/142#discussion_r688030472