Reduce N+1 queries when approvals by committers are not allowed

What does this MR do and why?

This reverts commit 6cf9fb71, reversing changes made to 92c318af.

Preload merge_request_diff_commits association of each MR when mergeRequests are queried via GraphQL.

We also need to batch load committers and lazy load the value of mergeable and detailed_merge_status to prevent N+1 queries for querying committers by their emails.

This also introduces a change to memoize MergeRequest#committers with arguments.

This is to prevent ArgumentError being raised when MergeRequest#committers was previously called with lazy: true and then called again with lazy: false but with a different expectation of return value (e.g. #select(:id) gets called on return value).

This also introduces lazy_merge_request_committers feature flag to only call CommitCollection#committers with lazy: true when enabled. This is ensure safety in case this results to errors and can easily be disabled.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Related to #441204 (closed)