Persist code quality payload mr diff

Ref: #267600 (closed)

This feature will be behind a feature flag called codequality_mr_diff

This is Step #3 toward creating a new codequality report attached to a pipeline.

What does this MR do?

This MR adds persists our codequality data required by the frontend for our feature.

It includes the following items:

• Creates new CodequalityMrDiff report
• Adds new Serializer to expose the required data used by the frontend
• Creates associated entity for it
• Persists the data as a pipeline artifact

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.

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