Build accessibility extension

What does this MR do and why?

Builds the accessibility widget as an extension behind feature flag refactor_mr_widgets_extensions

Existing widget app/assets/javascripts/reports/accessibility_report/grouped_accessibility_reports_app.vue

The new widget is on top in the screenshot the existing widget is on the bottom. We currently show both with the feature flag for review purposes.

Screenshots or screen recordings

How to set up and validate locally

1. Enable refactor_mr_widgets_extensions feature flag
2. Have a CI project setup
3. Follow the docs here to produce a job that has accessibility report
4. Create an MR to see the results of the job

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 #338275 (closed)