Support for impersonating service accounts #49
Description
I have a project that runs in google cloud, with the environment variable GOOGLE_APPLICATION_CREDENTIALS set appropriately. For local development, I don't have permission to generate those service account credentials and so I'd like to use an impersonated token, as described here.
Is this something that you'd be interested in supporting? If so, I have some code working that I'd be happy to clean up and contribute (but I also have some questions about it, and issues with the desired behavior not quite matching the TokenProvider trait...)