Add support for administrator in groups project_creation_level

Add support for administrator in groups project_creation_level

GitLab added support for administrator in project_creation_level for Groups as shown here:

gitlab!171245 (merged)

I'd like to request this option to be supported by the provider as well.

Related GitLab APIs

API documentation:

https://docs.gitlab.com/api/groups/

Additional Details

• GitLab REST API resources available (read for data sources, CRUD for resources)
• client-go supports the related GitLab API already
• I'd like to contribute it myself