Support local gcloud default credentials

I'm struggling to run grafana locally and use this data source. Ideally i'd like it to use my gcloud auth default credentials. The two configuration options at the moment look to be json (which i want to avoid) and gce (which isn't relevant locally).

It'd be great if this just transparently worked, like gcloud client libraries.

Start by locating the data source's current credential configuration and the code paths for the json and gce options. Compare those paths with the authentication behavior expected from local gcloud client libraries; done means local gcloud default credentials work without requiring a JSON key or GCE credentials.

Written by the indexing model from the issue text.