Add accessibility_reports endpoint to MergeRequest

Part of #211389 (closed)

This feature will be behind a feature flag called accessibility_report_view.

What does this MR do?

This MR creates a new accessibility_reports endpoint returning an accessibility report diff which will be displayed on a MR widget.

This is a standard approach to expose a new endpoint to our merge request controller. The implementation is similar to our existing features:

• test_reports
• coverage_reports

Why are we doing this?

Step #12 to introduce Accessibility Report comparison with our Merge Request Widget.

The entire feature has been developed on an MVC branch, this MR aims to break it down in smaller chunk.

This is the final step which close the loop of introducing this new feature behind our feature flag.

Screenshots

You can see how the feature will look like with the screenshot provided below:

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• [-] Separation of EE specific content

Availability and Testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

Dependencies

• !31369 (merged)