Ability to filter milestone by merge requests and vice versa

What does this MR do?

This merge requests adds two functionalities:

1. Ability to filter merge requests by milestones
2. Ability to retrieve all merge requests for a certain milestone

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

@rymai Shall I factor out the second functionality to a second merge request?

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #26301 (closed)