Refactor load performance widget to use extensions

What does this MR do and why?

This MR refactors the load performance widget to utilize MR Widget Extensions

Introduces refactor_mr_widgets_extensions and refactor_mr_widgets_extensions_user feature flags

Widget porting: ee/app/assets/javascripts/reports/load_performance_report/grouped_load_performance_reports_app.vue

Lower level: ee/app/assets/javascripts/vue_merge_request_widget/components/performance_issue_body.vue

Screenshots or screen recordings

Top is the new load performance widget using the extension, bottom is the existing load performance widget implementation.

How to set up and validate locally

Enable refactor_mr_widgets_extensions feature flag in rails console

Feature.enable(:refactor_mr_widgets_extensions)

Follow the docs https://docs.gitlab.com/ee/user/project/merge_requests/load_performance_testing.html#configure-the-load-performance-testing-job

OR

Setup the initLoadPerformanceReport loadPerformance property with the format below with two different load performance artifact paths. ee/app/assets/javascripts/vue_merge_request_widget/stores/mr_widget_store.js

this.loadPerformance = {
  base_path: '',
  head_path: '',
};

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #338287 (closed)