Support for feature flags (and feature flag user lists)

Created by: doriandekoning

Feature Description

It seems like feature flags and feature flag user lists are not supported yet, this would be immensely useful for us.

I could create a pull request myself to implement them.

Do you want to implement this?

• I would like to implement this myself 👷

Implementation Details

• Support is available in go-gitlab (https://github.com/xanzy/go-gitlab/blob/master/feature_flags.go)
• Support for CRUD operations is available within GitLab's API: https://docs.gitlab.com/ee/api/feature_flags.html

This should be a new resource: gitlab_project_feature_flag, implemented as a new Framework resource in internal/provider/resource_gitlab_project_feature_flag.go

Since the Read resource uses feature_flag_name, the flag name is what should be stored within the ID attribute of the resource.