Add PullMirrors::UpdateService

What does this MR do and why?

Contributes to #455520 (closed)

Problem

Pull mirror update is handled by Projects::Create/UpdateService. It's confusing and overcomplicated.

Solution

Add new service PullMirrors::UpdateService to handle pull mirror update procedure.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

1. Enable feature flag use_pull_mirror_update_service
2. Create a new pull mirror
3. Delete the pull mirror
4. Everything should work without any problems