Add finder for group-level jobs

What does this MR do?

As part of the effort to implement the changes proposed by #37366 (closed), I understand a new finder is required to fill a gap when managing Jobs on the group level. The goal of this MR is to add GroupJobsFinder, that will get all Jobs related to a given group, in all nested level of the Descendants groups.

• Regarding the GroupJobsFinder, it’ll get all jobs belonging to projects within a given group. The jobs can also be filtered by scope, if a params object is passed along with the call of the execute method.

This is the very first MR to address the issue and will be followed by more MRs addressing controller and view changes.

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

No, they follow the existing finders structure.

Does this MR meet the acceptance criteria?

• Documentation created/updated - N/A
• Added tests for this feature/bug
• In case of conflicts with master - no conflicts

What are the relevant issue numbers?