No ability to use GCP access tokens instead of credentials

[ntdesilv]

Hey, peeps!

It would be great to enable authentication to BigQuery using an access token rather than supplying credentials. Looks like this is available in the beta system at Google Cloud Dataproc:

Another alternative to passing the credentials, is to pass the access token used for authenticating the API calls to the Google Cloud Platform APIs. You can get the access token by running gcloud auth application-default print-access-token.
spark.read.format("bigquery").option("gcpAccessToken", "<acccess-token>")
or
spark.conf.set("gcpAccessToken", "<access-token>")

I'll start investigating and see if I can add a PR to this to enable this feature. Bit of a Scala rookie, though!