Add stubs for more channel credentials

[timw]

Adds stubs for:

• grpc.alts_channel_credentials
• grpc.alts_server_credentials
• grpc.compute_engine_channel_credentials
• grpc.insecure_server_credentials
• grpc.xds_channel_credentials
• grpc.xds_server_credentials

[shabbyrobe]

Thanks heaps! Really appreciate you taking the extra time to write some tests too, I usually have to add those myself. I will cut a new release now.