WIP: Add patch to create new merge requests via email

What does this MR do?

Allows creating new merge requests via email with a patch file.

Are there points in the code the reviewer needs to double check?

Applying patches from email were not supported before. This adds it via git am.

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered

What are the relevant issue numbers?

Closes #40830 (closed)