feat: Add ADC support for impersonated credentials

[sai-sunder-s]

1. Add support for loading an impersonated service account credential from a JSON (typically produced by running gcloud auth application-default login --impersoante-service-account <service account email>)
2. Support the JSON as part of ADC flow

Sample JSON:

{
  "delegates": [],
  "scopes": [
    "https://www.googleapis.com/auth/drive"
  ],
  "service_account_impersonation_url": "https://iamcredentials.googleapis.com/v1/projects/-/serviceAccounts/service-account-email@project-name.iam.gserviceaccount.com:generateAccessToken",
  "source_credentials": {
    "client_id": "oauth_client_id",
    "client_secret": "oauth_client_secret",
    "refresh_token": "user_refresh_token",
    "type": "authorized_user",
    "universe_domain": "googleapis.com"
  },
  "type": "impersonated_service_account"
}

[viacheslav-rostovtsev]

Please change the description. Main change is adding JSON file support to Impersonated Credentials, secondary change is enabling them in ADC flow. Scope override should not be in description .