gitserver: add GitserverLocalClone store
Created by: asdine
This adds a simple store that will be used to enqueue clone jobs between gitserver shards. The enqueue method will be used by several components:
- The OOB rendezvous migration, to migrate batches of repos
- The repository pinning feature, to trigger a migration whenever a repo is pinned
Test plan
Wrote a unit test
Related to #32827