Resolve "Add a `/create_merge_request` quick action"

What does this MR do?

With this quick action the user can create a new MR starting from the current issue using as source_branch the given branch name and as target_branch the project default branch. If the branch name is omitted a name is automatically created starting from the issue title.

For the reviewer

We don't deal with merge request creation errors, the main reasons are:

• we could launch multiple commands, some may error and some may not: dealing with multiple error messages/ errors only in some commands is quite complicated and I thought we could skip that for now
• we already don't handle some errors, for example when we /assign @invalid_user

Video example

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

Closes #40085 (closed)