Add FF for additional_repo_storage_by_namespace

Introduces the new feature flag additional_repo_storage_by_namespace
by adding the feature flag config file under EE.

ee/config/feature_flags/development/additional_repo_storage_by_namespace.yml

+---
+name: additional_repo_storage_by_namespace
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/43188
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/255166
+type: development
+group: group::fulfillment
+default_enabled: false