Add support for managing `Default description template for merge requests` property

Created by: radkomateusz

Hi there,

I want to submit feature request: Add support for managing Default description template for merge requests vie gitlab terraform provider.

Currently I don't see any option to mange it using terraform.

Option is under: Settings -> General -> Merge Request -> Default description template for merge requests

It could be done under gitlab_project resource or new resource could be added.

Thanks.