Return overridden property in API response

What does this MR do?

In order to determine whether a MR-level rule based on a project level rule is overridden (e.g. name, approvals required, users or groups have changed), a overridden property needs to be returned. This will be used by FE to determine which rule were overridden when shown on MR create and edit form.

Overridden rules will also be unscoped even if their source rule is scoped to protected branches.

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

#199795 (closed)