Add project_variable and group_variable datasources

Created by: caiconkhicon

Terraform Version

0.12.29

Expected Behavior

I want to save variables on GitLab and I can do it with gitlab_group_variable and gitlab_project_variable resources. However, I also want to use them by Terraform, thus I want to have variables as datasources. With the current GitLab provider, there is no such datasource. Please consider adding them in the next version of the provider.