feat: Add custom scopes for access tokens from the metadata service (#633)

This works for App Engine, Cloud Run and Flex. On Compute Engine you
can request custom scopes, but they are ignored.

Co-authored-by: Tres Seaver <tseaver@palladion.com>
Co-authored-by: Bu Sun Kim <8822365+busunkim96@users.noreply.github.com>
6 files changed