Implement gRPC endpoint to get the KAS version
For various reasons (see gitlab-org/gitlab#456414 (closed)) we need the KAS version on GitLab.
We've been using the KAS version from the GITLAB_KAS_VERSION
file in the monolith - however, with Introduces KAS release process (gitlab-org/release-tools!2860 - merged) it's also possible to have a plain commit SHA in that file - which is not semver compatible.
The idea of the RPC call is that the monolith can query KAS to just return a semver compatible version.
It shouldn't be a problem that when Rails calls this API a single random KAS will respond with its version - despite that e.g. a rollout is ongoing on some KAS already have a different version. This information is NOT mission-critical.