Allow backing up or restoring of repositories in a specific storage

What does this MR do and why?

#233296

Adds a new option REPOSITORIES_STORAGES to backup.rake. This accepts a comma separated list of storages, only these storages will be used to create or restore repositories.

How to set up and validate locally

1. Update gdk.yml to add more storages:

   gitaly:
     storage_count: 2

2. Rejigger gdk:

   $ gdk reconfigure && gdk restart

3. Move some repo to the new storage, it will be called gitaly-1.

   [1] pry(main)> move = Project.find(21).repository_storage_moves.build source_storage_name: 'default', destination_storage_name: 'gitaly-1'
   ...
   [2] pry(main)> move.schedule
   ...
   => true

4. Take a backup of only this new storage:

   $ bundle exec rake gitlab:backup:create REPOSITORIES_STORAGES=gitaly-1 SKIP=tar
   ...
   2022-05-10 14:43:52 +1200 -- Backup 1652150628_2022_05_10_15.0.0-pre is done.

5. Check that just that one repo was backed up. Restore works effectively the same. Create a full backup, and you can restore a specific repo.

   $ tree tmp/backups/repositories/
   tmp/backups/repositories/
   └── @hashed
       └── 6f
           └── 4b
               └── 6f4b6612125fb3a0daecd2799dfd6c9c299424fd920f9b308110a2c1fbd8f443
                   ├── 1652150628_2022_05_10_15.0.0-pre
                   │   ├── 001.bundle
                   │   ├── 001.refs
                   │   └── LATEST
                   └── LATEST
   
   5 directories, 4 files

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