Append inapplicable rules when creating MR

What does this MR do?

When creating an MR and overriding approval rules is enabled, we need to associate all the rules to the MR so they will still be available when MR's target branch changes.

This will be useful when FE starts to filter rules on MR create form as not all rules (only the applicable rules to target branch) will be submitted.

FE MR (still WIP as of this writing): !27971 (merged)

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

#199788 (closed)