Add support for using impersonate service account with gcloud cli

[rebaz94]

Fixes #454

[kevmoo]

Can you point me to docs that explain the impersonate values?

[rebaz94]

I can't find the docs for that but other client like Java already implemented this, you can check here.

When you run this command: gcloud auth application-default login --impersonate-service-account=principal@example.iam.gserviceaccount.com

it generate a file called application_default_credentials.json with the following content like below, and this PR just use the right value if type is impersonated_service_account

{
  "service_account_impersonation_url": "",
  "source_credentials": {
    "client_id": "",
    "client_secret": "",
    "refresh_token": "",
    "type": "authorized_user"
  },
  "type": "impersonated_service_account"
}

[kevmoo]

@rebaz94 – is there an issue open for this?

[rebaz94]

@rebaz94 – is there an issue open for this?

yes here #454