Implement BE for "MR feedback" (creating an MR from a remediation)

What does this MR do?

This MR provides backend support to ensure "MR feedback" (creating an MR from a remediation) works as expected on the new Standalone Vulnerabilities page (#216300 (closed)).

Per @aturinske:

So, as of right now, the FE needs:

• the create_vulnerability_feedback_merge_request_path endpoint on the vulnerability hash
• the target_branch on the vulnerability hash, which seems to coming from ee/app/views/projects/pipelines/_tabs_content.html.haml as source_branch for the modal
• the remediation on the finding hash

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