feat: support google service account for gke usage

[mabdh]

• Support GKE-specific auth
• Add 1 k8s config called provider_type
• If provider_type does exist
  • The other auth flows like Token and Client Cert would be ignored
  • Only support gke type at the moment
  • Entropy will try to do auth with FindDefaultCredentials

FindDefaultCredentialsWithParams searches for "Application Default Credentials".
It looks for credentials in the following places, preferring the first location found:

• A JSON file whose path is specified by the GOOGLE_APPLICATION_CREDENTIALS environment variable. For workload identity federation, refer to https://cloud.google.com/iam/docs/how-to#using-workload-identity-federation on how to generate the JSON configuration file for on-prem/non-Google cloud platforms.
• A JSON file in a location known to the gcloud command-line tool. On Windows, this is %APPDATA%/gcloud/application_default_credentials.json. On other systems, $HOME/.config/gcloud/application_default_credentials.json.
• On Google App Engine standard first generation runtimes (<= Go 1.9) it uses the appengine.AccessToken function.
• On Google Compute Engine, Google App Engine standard second generation runtimes (>= Go 1.11), and Google App Engine flexible environment, it fetches credentials from the metadata server.