Fix rubocop errors by adding type annotations

What does this MR do?

This adds type annotations to user merge request resolvers, and renames the common base class so it is clear it is an abstract base class, and should not be used as an actual resolver.

See: #281697 (closed)

As a developer-facing change, with no functional impact, this does not require a changelog.

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.