Add project scope CI lint API endpoint

What does this MR do?

Adds the /api/v4/projects/:id/ci/lint endpoint which lints the project CI config file.

Related #231352 (closed)

Does this MR meet the acceptance criteria?

Conformity

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