Soft-warn about application default credentials using gcloud credentials

Due to user issues with quota and API enablement, the auth libraries should issue a soft warning (that can be silenced) when application default credentials uses user credentials from the Cloud SDK. They should not warn if the credentials for the cloud sdk are service account credentials.

Your application has authenticated using end user credentials from Google 
Cloud SDK. We recommend that most server applications use service accounts
 instead. If your application continues to use end user credentials from Cloud 
SDK, you might receive a "quota exceeded" or "API not enabled" error. For 
more information about service accounts, see 
https://cloud.google.com/docs/authentication/.

Additional context: https://groups.google.com/a/google.com/forum/#!topic/client-auth-team/DKqrFw6lL1Q https://buganizer.corp.google.com/issues/64388723 https://github.com/GoogleCloudPlatform/google-auth-library-python/pull/266

Note: It’s unclear to me if this belongs here, or in the apiary client repo. Feel free to move if needed 😄

[jskeet]

Personally I still think the best solution is for the server to give a better error message. There simply isn’t a general warning mechanism to tie into in .NET. I don’t think we should be making any changes client-side here.

[jskeet]

@theacodes: In reality it would often be end users rather than the developers, which makes it trickier. (But yes, a side-channel for warnings in general might be useful.)

I think so long as it starts failing with a good error message, I think that’s okay. We can put up prominent docs etc.