Cache merge request diff version API

Due to the Git's immutable nature merge request diff versions
stay the same. We can go ahead and cache them.

changelogs/unreleased/id-cache-cache-merge-request-versions-version-api.yml

+---
+title: Cache merge request diff version API
+merge_request: 57568
+author:
+type: performance

config/feature_flags/development/cached_api_merge_request_version.yml

+---
+name: cached_api_merge_request_version
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/57568
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/326093
+milestone: '13.11'
+type: development
+group: group::source code
+default_enabled: true

lib/api/merge_request_diffs.rb

@@ -45,8 +45,12 @@ module API
 
         merge_request = find_merge_request_with_access(params[:merge_request_iid])
 
+        if Feature.enabled?(:cached_api_merge_request_version, default_enabled: :yaml)
+          present_cached merge_request.merge_request_diffs.find(params[:version_id]), with: Entities::MergeRequestDiffFull, cache_context: nil
+        else
           present merge_request.merge_request_diffs.find(params[:version_id]), with: Entities::MergeRequestDiffFull
         end
       end
     end
+  end
 end

spec/requests/api/merge_request_diffs_spec.rb

@@ -75,5 +75,25 @@ RSpec.describe API::MergeRequestDiffs, 'MergeRequestDiffs' do
         let(:url) { "/projects/#{project.id}/merge_requests/#{merge_request.iid}/versions/#{merge_request_diff.id}" }
       end
     end
+
+    context 'caching merge request diffs', :use_clean_rails_redis_caching do
+      it 'is performed' do
+        get api("/projects/#{project.id}/merge_requests/#{merge_request.iid}/versions/#{merge_request_diff.id}", user)
+
+        expect(Rails.cache.fetch(merge_request_diff.cache_key)).to be_present
+      end
+
+      context 'when cached_api_merge_request_version is disabled' do
+        before do
+          stub_feature_flags(cached_api_merge_request_version: false)
+        end
+
+        it 'is not performed' do
+          get api("/projects/#{project.id}/merge_requests/#{merge_request.iid}/versions/#{merge_request_diff.id}", user)
+
+          expect(Rails.cache.fetch(merge_request_diff.cache_key)).to be_nil
+        end
+      end
+    end
   end
 end