fetch_id_token not reading gcloud creds

[sai-sunder-s]

fetch_id_token checks only GOOGLE_APPLICATION_CREDENTIALS env var. It is not checking the well-known location (.config/gcloud/application_default_credentials.json) where gcloud writes the credential.
Ideally we should check both locations like how the Access Token Cred ADC does here.

Due to this bug, if user ran gcloud auth application-default ... command and runs id_token.fetch_id_token, the cred is not picked as expected.

[parthea]

Transferring to google-cloud-python as the code for google-auth-library-python will be migrated there in 1-3 weeks.