Support ADC json when impersonating service user

[bailer]

Is your feature request related to a problem? Please describe.
When generating an ADC json you are able to impersonate a service user which outputs a JSON file that is not currently handled.
e.g.

gcloud auth application-default login --impersonate-service-account=server@example.iam.gserviceaccount.com

Which generates the following JSON format:

{
  "delegates": [],
  "service_account_impersonation_url": "https://iamcredentials.googleapis.com/v1/projects/-/serviceAccounts/server@example.iam.gserviceaccount.com:generateAccessToken",
  "source_credentials": {
    "client_id": "1234567890.apps.googleusercontent.com",
    "client_secret": "<secret>",
    "refresh_token": "<refrest_token>",
    "type": "authorized_user"
  },
  "type": "impersonated_service_account"
}

Currently the library throws the following error when doing this:
The incoming JSON object does not contain a client_email field
Describe the solution you'd like
I'd like the library to detect this format and handle it correctly
Describe alternatives you've considered
I can manually alter the file by putting the contents of source_credentials at the top level of the JSON (though unsure if this has any side-effect). I can also generate a key for the service user but since it's not recommended, impersonating the service user and using ADC would be preferred.
Additional context
I'm very new to GCP and authentication in that regard and was trying to set up local development without generating keys but still using a service user. I found that you can impersonate a service user while generating the ADC JSON but it's not working with this library. I found that e.g. google-auth-library-python supports this format so I'm thinking the node library also should.

[imathews]

Just a friendly +1 on this — I'm running into the same issue with google-auth-node, which is preventing us from utilizing impersonated service accounts in our local dev environment.

[danielbankhead]

This has been resolved via #1425