Add support for feature flags

Add two new Framework resources implementing support for GitLab project feature flags and their user lists (#1213).

gitlab_project_feature_flag manages a project-level feature flag's name, description, active state, and rollout strategies. Changing strategies replaces the resource: the GitLab API removes a strategy via a _destroy flag that client-go's typed strategy struct doesn't expose, so an in-place update could only add strategies, never remove ones no longer in the config. The gitlabUserList strategy is rejected via ValidateConfig for the same reason (it needs a user_list_id field client-go doesn't expose).

gitlab_project_feature_flag_user_list manages the named user lists themselves (name + comma-separated external user IDs), which can be assigned to a gitlabUserList strategy outside of Terraform.

Both resources use client-go v2.47.0's native ProjectFeatureFlags and FeatureFlagUserLists services directly; no raw HTTP calls or forked dependencies are needed.