Compute: validate the use of gRPC service config before the next release

[jskeet]

#16751 regenerated Compute using the gRPC service config to provide timeouts etc. This should be fine (it's used in all other languages), but we should validate it before we next do a release.

This issue will be used as a comment in .librarian/config.yaml for blocking the release of compute until we're happy.

[parthea]

We should consider creating a pre-release version of google-cloud-compute since the library is stable. We should also ship google-cloud-compute-v1beta first (regardless of pre-release since it's preview)