auth: identity token source

[gwik]

Adds a token source to fetch a JWT token from the metadata server for a specific audience.

See:

https://cloud.google.com/run/docs/authenticating/service-to-service#use_the_metadata_server

[antoninfin]

Here's a modified version based on the idtoken go package.

I'll document and test it a little bit.

[yoshidan]

Thanks!