Allow to add patches to merge requests created via email

What does this MR do?

This allows users to add patches as attachments to merge request created via email.

When an email to create a merge request is sent, all the attachments ending in .patch will be applied to the branch specified in the subject of the email. If the branch did not exist, it will be created from the HEAD of the repository.

When the patches could not be applied, the error message will be replied to the user.

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/40830

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.