Change gRPC channel configuration to include `DEFAULT_AUTHORITY`

[coryan]

Channels intended for storage.googleapis.com should set:

args.SetString(GRPC_ARG_DEFAULT_AUTHORITY, "storage.googleapis.com");

I am not sure if this is only needed when using Google Direct Access (i.e. google-c2:///* endpoints), or a general thing.

[scotthart]

This could apply to all grpc libraries, needs investigation.