Add preloading to MergeRequest#committers

What does this MR do and why?

Part of a process to begin preloading merge request commit metadata to avoid possible N+1 queries. This will consist of 5 MRs total, described in the table below.

This MR uses the preload_commits_metadata method added in Add preload option to MergeRequest#recent_commi... (!204107 - merged) to also add preloading to the committers method.

Order	MR/status	Description
Part 1	!203465 (merged)	Adding scope to preload the latest diff commit
Part 2a	!204107 (merged)	Add preloading to MergeRequest#recent_commits
Part 2b 📍	!204128 (merged)	Add preloading to MergeRequest#committers (relies on !204107 (merged))
Part 3	!204132 (merged)	Add preloading to MergeRequestDiff#commit_shas
Part 4	!204123 (merged)	Add Banzai find_commit_by_sha preloading

References

https://gitlab.com/gitlab-org/gitlab/-/issues/527235

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