Create PackObjectsHook RPC in Gitaly, without caching

• Create the hook so that we can gather data about what we will be caching.
• Roll out to gitlab-org/gitlab server.

Development:

• Make git-upload-pack use the gitaly-hooks executable when it wants to run pack-objects. No RPC calls or caching yet. gitlab-org/gitaly!3038 (merged)
• Add PackObjectsHook RPC and make gitaly-hooks call it gitlab-org/gitaly!3084 (merged)
• Register feature flag for hook in gitlab-org/gitlab gitlab-org/gitlab!53132 (merged)

Outcomes:

• Observe CPU / memory overhead of having the hook
• Reliably observe number of requests (SSHUploadPack and PostUploadPack over-count)
• Log would-be cache keys for hook requests, so we can predict cache hit ratios
• Log bytes served so we can predict storage and IO needs