Extract shared functionality from ProjectTreeRestorer

What does this MR do?

This MR is part of a bigger MR !19560 (closed) that introduces Group Import functionality (similar to Project Import what we have today). Mentioned MR is going to be split into multiple MRs in order to make it easier to review and merge.

This MR extracts ProjectTreeRestorer functionality into a new class RelationTreeRestorer in order to be used in a new GroupTreeRestorer that is going to be added in a future MR. It's done in order to share as much functionality as possible with Project Import & reduce duplication, since the process of Import for both project & group are more or less the same.

This MR also changes a few other classes in order for them to become compatible with not only projects, but also groups.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

N/A

Security

N/A