Refactor merge request association preloader to be in a SSOT

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This is based on @hfyngvason's suggestion in https://gitlab.com/gitlab-org/gitlab/-/issues/514744#note_2325818199 about having a SSOT class/module that will include all preloaded associations that we can use in different places. This way, we can re-use it and ensure it applies to all places it is needed.

This should include shared examples or helper methods that can be re-used by consumers.

As of this writing (2025-03-19), there are following preloaders in different locations:

• ee/app/finders/ee/merge_requests_finder.rb
• ee/app/graphql/resolvers/concerns/ee/resolves_merge_requests.rb
• ee/app/models/ee/merge_request.rb
• ee/lib/ee/gitlab/ci/variables/preloader/merge_request.rb
• (Upcoming) ee/lib/ee/gitlab/data_builder/pipeline.rb