How should google-cloud-auth CacheableResource<> be used?

Excited to have an official Rust SDK, so thanks!

I'm trying to figure out how to use the CacheableResouce<> type in google-cloud-auth and have no idea how I'm supposed to use the returned `EntityTag` to cache the returned auth headers. Is it deliberate that there's no user-visible timeout on the auth headers? Or is there an API that I'm supposed to be able to use the `EntityTag` with to determine if my headers are still sufficiently recent (I don't see anything on CredentialsProvider that I can use)?

Links:
- https://docs.rs/google-cloud-auth/1.0.0/google_cloud_auth/credentials/struct.Credentials.html
- https://docs.rs/google-cloud-auth/1.0.0/google_cloud_auth/credentials/trait.CredentialsProvider.html
- https://docs.rs/google-cloud-auth/1.0.0/google_cloud_auth/credentials/enum.CacheableResource.html