Imagestore: Fallback to application default credentials when no key file is specified for GCS (#25948)

The external image storage for GCS creates the JWT Token from a credentials file, 
but if your Grafana server runs under a GCE instance with a service account on it, 
you can use that instead (you don't have to manage/secure the credentials file).

Co-authored-by: Arve Knudsen <arve.knudsen@gmail.com>
Co-authored-by: Emil Tullstedt <emil.tullstedt@grafana.com>