Allow restore tasks to be skipped using SKIP

What does this MR do and why?

Fixes #19347 (closed)

Allows skipping specific restore tasks when restoring a backup. This allows for a more selective restore when restoring everything hasn't worked.

How to set up and validate locally

1. Create a backup

   $ bundle exec rake gitlab:backup:create
   ...
   2022-03-29 10:50:50 +1300 -- Backup 1648504234_2022_03_29_14.10.0-pre is done.

2. Restore with some tasks skipped. Restore doesn't log anything when a task is skipped. So the absence of these tasks indicates success here.

   bundle exec rake gitlab:backup:restore BACKUP=1648504234_2022_03_29_14.10.0-pre SKIP=repositories,db
   2022-03-29 10:52:52 +1300 -- Unpacking backup ... 
   2022-03-29 10:52:53 +1300 -- Unpacking backup ... done
   2022-03-29 10:52:53 +1300 -- Restoring uploads ... 
   2022-03-29 10:52:53 +1300 -- Restoring uploads ... done
   2022-03-29 10:52:53 +1300 -- Restoring builds ... 
   2022-03-29 10:52:53 +1300 -- Restoring builds ... done
   2022-03-29 10:52:53 +1300 -- Restoring artifacts ... 
   2022-03-29 10:52:53 +1300 -- Restoring artifacts ... done
   2022-03-29 10:52:53 +1300 -- Restoring pages ... 
   2022-03-29 10:52:53 +1300 -- Restoring pages ... done
   2022-03-29 10:52:53 +1300 -- Restoring lfs objects ... 
   2022-03-29 10:52:53 +1300 -- Restoring lfs objects ... done
   2022-03-29 10:52:53 +1300 -- Restoring terraform states ... 
   2022-03-29 10:52:53 +1300 -- Restoring terraform states ... done
   2022-03-29 10:52:53 +1300 -- Restoring packages ... 
   2022-03-29 10:52:53 +1300 -- Restoring packages ... done
   ...

MR acceptance checklist

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