Migrate `resource_group` configuration feature to use python-gitlab library

[amimas]

Goal of this issue is to migrate the resource_groups configuration feature to use python-gitlab library instead of using homegrown API in this project.

Todo:

• Delete gitlabform/gitlab/resource_groups.py as it won't be needed
• Update gitlabform/processors/project/resource_groups_processor.py to use python-gitlab library