REST: Add runner_manager field to API::Entities::Ci::Job

What does this MR do and why?

REST: Add runner_manager field to API::Entities::Ci::Job

Currently, the job entity contains a runner field. However, this field is no longer sufficient to determine where a job actually ran, For that, we'll need to add a runner_manager field to the entity.

Changelog: added

How to set up and validate locally

1. set up a job to build a project
2. run curl --globoff --header "PRIVATE-TOKEN: <glpat token>" http://gdk.test:3000/api/v4/projects/<projectid>/jobs
3. observe that the result now contains a runner_manager property

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.

Related to #415444