Add codequality_mr_diff feature flag

This feature flag  will be used to display codequality diff report
in a merge request.

config/feature_flags/development/codequality_mr_diff.yml

+---
+name: codequality_mr_diff
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/47938
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/284140
+milestone: '13.7'
+type: development
+group: group::testing
+default_enabled: false