Add id_token capability on User Credential

[guillaumeblaquiere]

To call private Cloud Run and Cloud Functions, we need to add a valid id_token. The library doesn't allow to generate one (not implement IdTokenProvider interface.)

The gcloud command, with the same user credential, allows to generate an id_token. gcloud auth print-identity-token

Problem

When I develop locally, I have to generate and download a service account key file to be able to tests private services from my code.

Service account key file is a nightmare for security (I wrote an article on this)

Expected behavior

To have the capability to generate a valid signed token from google auth library as the gcloud SDK do.