Implement diff_file endpoints

What does this MR do and why?

Implement diff_file endpoints

This adds diff_file endpoint for each resource that we support in rapid diffs (e.g. MergeRequest, Commit and Compare).

This is behind rapid_diffs FF.

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable the rapid diffs feature flags:

Feature.enable(:rapid_diffs)
Feature.enable(:rapid_diffs_on_mr_creation)
Feature.enable(:rapid_diffs_on_mr_show)
Feature.enable(:rapid_diffs_on_commit_show)
Feature.enable(:rapid_diffs_on_compare_show)

2. You can use a project that has a README.md so it's easy to append rapid_diffs=true&old_path=README.md&new_path=README.md the below URLs to test

3. Test the endpoint for each resource:

   • /projects/:project_id/merge_requests/:merge_request_id/diff_file?
   • /projects/:project_id/commit/:id/diff_file?
   • /projects/:project_id/merge_requests/new/diff_file?
   • /projects/:project_id/compare/diff_file?from=:version&to=:version&

MR acceptance checklist

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

Related to #506168 (closed)