Resolve "Improve performance of Group Search API (advanced): merge_requests scope"

What does this MR do?

This MR adds some batch loaded attributes to the API::Entities::MergeRequestBasic to eliminate some N+1 queries that are done whenever the entity is being serialized without having its relationships preloaded.

By doing this work in the entity, we eliminate the need to eager load the associations if they are not used anywhere else but by the entity serializer.

Screenshots

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

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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

Closes #221211 (closed)