Resolve "Hashed Storage: Make possible to migrate single project"

What does this MR do?

• Refactor migration scheduler to concentrate important code into single service class
• Improve specs related to storage migration
• Make possible to migrate single project (to help with our gitlab.com migration: https://gitlab.com/gitlab-com/infrastructure/issues/4174#note_74443154)

Are there points in the code the reviewer needs to double check?

I'm unsure about refactoring the log part as well. I could re-use sidekiqs log and have all information there vs parts in sidekiq.log and production.log

Why was this MR needed?

We are trying small rollouts before doing the big one, see https://gitlab.com/gitlab-com/infrastructure/issues/4174

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a Backend maintainer
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #46922 (closed)