Create feature flag for Gitaly pack object hook

This exists just to register a feature flag in Gitaly that we'll want to
control for Rails.

config/feature_flags/development/gitaly_upload_pack_gitaly_hooks.yml

+---
+name: gitaly_upload_pack_gitaly_hooks
+introduced_by_url:
+rollout_issue_url: https://gitlab.com/gitlab-com/gl-infra/scalability/-/issues/807
+milestone: '13.9'
+type: development
+group:
+default_enabled: false