gRFC L69: Allow Call Credentials to be Specified in grpc_google_default_credentials_create

[Richard Belleville]

All,

I've drafted a proposal to change the constructor for google default credentials in the Core API to accommodate a corner case that is not currently served well by the API. Please take a look.

Thanks,

Richard Belleville

[Richard Belleville]

I've just realized that L69 is already taken by a proposal in the draft stage. I've updated the proposal title to "L69: Allow Call Credentials to be Specified in grpc_google_default_credentials_create". Pardon the mistake.