Draft: POC: Use backend codequality diff for MR widget

What does this MR do?

Proof of concept for frontend work to use the new backend code quality diff for the code quality MR widget

I'll split this into many MRs and keep this table updated:

MR	What's it do?
!50986 (merged)	expose new endpoint on MR widget entity, push feature flag to frontend
!52365 (merged)	use the backend code quality diff when the feature flag is enabled
!54241 (merged)	enable the feature flag by default
!55708 (merged)	remove the feature flag related code on the frontend
?	documentation?

Screenshots (strongly suggested)

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

Related to #271077 (closed)