Add "prevent forking project outside of group" attribute to "gitlab_group" resource

As the title states there is an option in GitLab to prevent the forking of projects outside of the group that owns them.
The gitlab_group resouce would have to be modified to add this in.

applicable license tier's

• premium
• silver

Relevant Links:

• https://docs.gitlab.com/ee/user/group/#prevent-project-forking-outside-group