Filter rules by target_branch in approval_settings

What does this MR do?

With scoped approval rules, it's possible that all approval rules does not apply to an MR's target branch.

When changing target branch of an MR, we need to know the rules that will be applicable for the MR before it changes. This adds that support via target_branch param.

This is needed by the frontend so it can display approval rules in MR create/edit form based on the selected target branch.

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) and #199790 (closed)