Scopeless credentials

[nathanielmanistaatgoogle]

@anthmgoogle, @jboeuf, and possibly @tbetbetbe will specify.

[dhermes]

These already exist to some degree since most Credentials subclasses implement create_scoped(scopes) and create_scoped_required()

---

I await what they specify.

[jboeuf]

I'm a little confused here. Is this the JWT Access credentials that is missing or the plumbing to default application credentials for gRPC?

[dhermes]

I've been using a service account and adding an access token to the auth header for the Bigtable implementation (when creating a stub).

[jboeuf]

It's not quite the same thing. I'll send you the doc offline.

[nathanielmanistaatgoogle]

@jboeuf directs that this is a duplicate of #252.