Add caching around widget JSON [RUN ALL RSPEC] [RUN AS-IF-FOSS]

app/views/projects/merge_requests/_widget.html.haml

-- artifacts_endpoint_placeholder = ':pipeline_artifacts_id'
+= cache_if(Feature.enabled?(:cached_mr_widget, @merge_request.project), [@merge_request.project, @merge_request, current_user], expires_in: 10.minutes) do
+  - artifacts_endpoint_placeholder = ':pipeline_artifacts_id'
 
-= javascript_tag do
+  = javascript_tag do
     :plain
       window.gl = window.gl || {};
       window.gl.mrWidgetData = #{serialize_issuable(@merge_request, serializer: 'widget', issues_links: true)}
@@ -18,4 +19,4 @@
       window.gl.mrWidgetData.pipelines_empty_svg_path = '#{image_path('illustrations/pipelines_empty.svg')}';
       window.gl.mrWidgetData.codequality_help_path = '#{help_page_path("user/project/merge_requests/code_quality", anchor: "code-quality-reports")}';
 
-#js-vue-mr-widget.mr-widget
+  #js-vue-mr-widget.mr-widget

config/feature_flags/development/cached_mr_widget.yml

+---
+name: cached_mr_widget
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/61584
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/330803
+milestone: '13.12'
+type: development
+group: group::code review
+default_enabled: false