feat: Integrate Repositories::CommitsFinder with GitalyKeysetPager

Why

Add Repositories::CommitsFinder (created in !235257 (merged)) case to Gitlab::Pagination::GitalyKeysetPager.

Put case behind new feature flagcommits_keyset_pagination (default off) since this is part of a series of MRs and it's not ready to turn on yet.

What

• Add the commits_keyset_pagination feature flag (gitlab_com_derisk, group::source code, default off).
• Add a when ::Repositories::CommitsFinder branch to GitalyKeysetPager#keyset_pagination_enabled? behind new feature flag
• Add tests

Notes

• CommitsFinder is intentionally keyset-only. It is not added to paginate_with_offset_headers? because it does not implement total
• Endpoint wiring is deferred to #595508. This MR only makes the pager recognize the finder.

Part of #595507 (closed)

Part of the epic: gitlab-org#21573