Add GitDeduplicationService

What does this MR do?

fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/60871

This service performs a number of housekeeping tasks for deduplicated projects.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry (developer facing change)
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content