WIP: Remove `RelationFactory::OVERRIDES`, use reflection instead

What does this MR do?

This removes OVERRIDES and prefers to use relations to reflect classes.

This also performs a strong validation of all relations, if the can be restored.

There are a bunch of decisions to make with invalid individual objects.

This makes the RelationFactory and ProjectTreeRestorer to be very close to be "Generic" RelationFactory and TreeRestorer where the Project is one of the usage patterns, as this removes a lot of very project specific code from RelationFactory.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team