Preload merge request diffs for Throughtput table

What does this MR do?

This MR preloads merge requests diffs on the DiffStatsSummary field of MergeRequest type (GraphQL), in order to reduce the number of queries that occur with loading the Throughput table on MR Analytics page.

This is towards #243709 (closed)

Example GraphQL query

{
  project(fullPath:"gitlab-org/gitlab-test") {
    mergeRequests {
      nodes {
        id
        diffStats {
          additions
          deletions
        }
      }      
    }
  }
}

Before this change the query above generates N+1 queries on the merge_request_diffs and the merge_request_diff_commits tables.

Database query

We expect maximum 100 merge request ids (max items per page in GraphQL)

SELECT "merge_request_diffs".*
FROM "merge_request_diffs"
INNER JOIN
  (SELECT DISTINCT FIRST_VALUE(id) OVER (PARTITION BY merge_request_id
                                         ORDER BY created_at DESC) AS id
   FROM "merge_request_diffs"
   WHERE "merge_request_diffs"."merge_request_id" IN (74672729,
                                                      74671284,
                                                      74670720,
                                                      74670050,
                                                      74667320,
                                                      74666241,
                                                      74665211,
                                                      74664673,
                                                      74664391,
                                                      74659727,
                                                      74659453,
                                                      74658415,
                                                      74657110,
                                                      74656781,
                                                      74656119,
                                                      74654621,
                                                      74653759,
                                                      74653405,
                                                      74653076,
                                                      74652570)) latest_diffs ON latest_diffs.id = merge_request_diffs.id

Plan: https://explain.depesz.com/s/X0zY

Does this MR meet the acceptance criteria?

Conformity

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

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team