Pull mirror GET API

What does this MR do and why?

Contributes to #354506 (closed)

Based on !99519 (closed), thank you @markus.ferrell! 🙇

Problem

GitLab API has an endpoint to fetch push mirroring status, but this functionality is missing for pull mirrors.

Solution

Add Pull mirroring API endpoint to return information about pull mirroring status.

Screenshots or screen recordings

How to set up and validate locally

1. Setup a pull mirror for the project (see docs)
2. Visit http://127.0.0.1:3000/api/v4/projects/_project_id_/mirror/pull
3. You should see the response with necessary information about pull mirrors

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.