Support reftable migration using git refs migrate --dry-run

Following the plan outlined, add changes to Gitaly to enable rollout of reftable without the WAL.

This includes:

1. Note the last modified time for the existing packed-refs file
2. Run git refs migrate --dry-run
3. Apply all loose refs present in the repository to the reftable dry-run folder
4. If packed-refs isn't modified
   1. Block all writes incoming to the repository
   2. Commit the reftable dry-run folder and modify the repository config to use reftable
   3. Unblock the repository
5. If packed-refs is modified, delete the dry-run folder and restart from #1