Pull mirror API: extend response with mirror related fields

What does this MR do and why?

Contributes to #494294

Problem

Existing endpoint does not return information about mirror configuration state. It's not possible to see mirror settings using Pull mirror API.

Solution

Expose missing settings.

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.

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