Add support for inline credentials via credentials argument in connection config

[johnsmyth]

Is your feature request related to a problem? Please describe.
I would like to be able to specify the gcp credentials inline in the config file, not in an external json file.

Describe the solution you'd like
Add support for a credentials argument in the connection config to allow a user to specify either the path to or the contents of a service account key file in JSON format. This should look roughly like the credentials arg in the Terraform google provider

After this is implemented, we may want to deprecate credential_file as it would be redundant at that point.