resource/gitlab_project: support forking a project

This change set introduces a new attribute fork_from_project_id which can be used to create a new project by forking an existing one.

The attribute mr_default_target_self is not yet supported, because it's lacking in go-gitlab right now, see https://github.com/xanzy/go-gitlab/pull/1590. This MR already brings enough value and we can add it once it's available.

Closes: #98 (closed)

It will also allow us to continue Dogfooding here: gitlab-org/frontend/renovate-gitlab-bot!607 (merged)