Add reliable sidekiq fetcher

What does this MR do?

There is abandoned project https://github.com/TEA-ebook/sidekiq-reliable-fetch which does not work with recent Sidekiq.

I created a repo for the fork https://gitlab.com/gitlab-org/sidekiq-reliable-fetch

I updated the gem and used it here.

Besides the just working reliable fetcher, we also need to integrate feature flipper and make sure we can monitor everything.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/7279

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Add someone from GitLab as an owner for the gem on rubygems