Register the snapshot_archiving feature flag

Default off. Consumed by graylog-plugin-enterprise, which gates the Snapshot Archiving config endpoint and UI on it. An unregistered flag already reads as off, so this can merge on its own.

Author: fpetersen-gl

graylog2-server/src/main/resources/org/graylog2/featureflag/feature-flag.config

@@ -135,3 +135,6 @@ replay_search_right_sidebar=on
 
 # Show predefined table layouts for alerts data table
 show_alerts_predefined_layouts=on
+
+# Enterprise: archive indices from OpenSearch snapshots instead of the scroll path
+snapshot_archiving=off