Support Workload Identity (getting access tokens from the metadata server)

[anna-hope]

The recommended way for applications in GKE to access GCP services is now via Workload Identity (WI). This replaces the need to use service accounts to provide credentials, with the token obtained from the GKE metadata server.

Are there plans for this crate to support this mode of authentication? As mentioned in #92, gcp_auth is a good candidate to handle auth. Its documentation states that it supports getting the token from the metadata server.

Are there plans to work on #92, or support WI auth in some other way? I looked through the code of this crate, but couldn't find a clear place where alternative auth methods could be slotted in.