Add codequality mr diff report endpoint

Ref: #267600 (closed)

This feature will be behind a feature flag called codequality_mr_diff

Final Step #4 toward creating a new codequality report attached to a pipeline.

What does this MR do?

This MR creates a new endpoint to expose our new codequality report used on mr diff.

It includes the following items:

• Adds new route
• Adds new controller action
• Creates new presenter to select only modified files on the merge request
• Adds model methods

A pipeline artifact is used to persist generated report after pipeline completion.

For example we use it for displaying tested code on a MR diff. We will start using this abstraction for displaying codequality warning on diff/MR.

Why are we doing this?

The grouptesting is currently planning to build up more features with codequality and one of those is to display codequality diff on the MR itself. This will be similar to our current test coverage feature.

This feature is being built to display codequality degradation on MR diff. ⚠ This is different than our existing codequality feature on MR widget.

Screenshots

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