Unconditionally add AuthTransport.

Today, endpoints implementing v2 cannot properly fallback to v1 because the underlying transport that deals with authentication (Basic / Token) doesn't get annotated.

This doesn't affect DockerHub because the DockerHub endpoint appears as 'https://index.docker.io/v1/' (in .dockercfg), and the 'v1' tricks this logic just long enough that the transport is always annotated for DockerHub accesses.

Signed-off-by: Matt Moore <mattmoor@google.com>