LRU object caching for GroupProjectObjectBuilder

What does this MR do?

LRU object caching for GroupProjectObjectBuilder, for issue #39112 (closed)

Introduce LRU cache to GroupProjectObjectBuilder. This assumes Labels/Milestones will not change during project import, once they are created in database.

It reduces database queries and object creation for labels/milestonesduring project import.

• reduces import time when project heavily uses labels/milestones
• reduces database pressure

Conformity

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

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