Utilize the migration framework to add refs backend migration

Once the migration framework for Gitaly + transaction lands, we'll have the necessary code to enable different migrations.

For us to be able to migrate to reftables, we need to utilize the framework to add a reference backend migration job. The job must:

• Note the current state of the repository, by obtaining a hash of the refs using git for-each-ref --include-root-refs
• Run git refs migrate --ref-format=reftables to migrate to reftables
• Verify correctness by recomputing the hash of the refs and comparing
• Aborting the transaction if any of the steps fail