Add codequality backend comparison feature flag [RUN ALL RSPEC] [RUN AS-IF-FOSS]

Ref: #271077 (closed)

What does this MR do?

This MR creates a new feature flag which will be used to toggle our codequality feature comparison on the backend.

Why are we doing this?

A month ago, we introduced codequality_mr_diff feature flag which will actually be used for a different feature.

In order to make sure we can move the comparison logic from the frontend to the backend smoothly we decided to create a separate feature flag for it called codequality_backend_comparison.

This MR updates the relevant codepaths for our feature.

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