Add support for creating sub-tasks

Hello,

It appears that it's not currently possible to add a sub-task (or child item) to an existing issue. This feature would be very helpful for breaking down larger tasks into smaller, more manageable pieces.

Expected Behavior: It should be possible to create sub-tasks that are linked to a parent issue and displayed hierarchically under the main issue.

Current Behavior: This functionality is not currently available.

Would it be possible to add this feature to the gitlab-mcp server?

Thank you for your support.