Fix N+1 issues in /api/v3/repos/:namespace/:project/events [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

The /api/v3/repos/:namespace/:project/events endpoint was flagged as having N+1 problems #321839 (closed).

The N+1 issues happen when a collection of MergeRequests are serialised as API::Github::Entities::PullRequest entities.

The entity loads these associations:

• assignee
• author
• metrics
• source_project These should all be the same but including it avoids relying on the query cache
• target_project

This MR applies eager loading to avoid N+1 issues during serialization.

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

Related to #321839 (closed)