Add a feature flag for reindex resilience (#138618)

This is not used yet, but several related streams of work will be
hidden behind it until they are ready for production.

Closes elastic/elasticsearch-team#2084

Author: PeteGillinElastic

modules/reindex/src/main/java/org/elasticsearch/reindex/ReindexPlugin.java

@@ -19,6 +19,7 @@
 import org.elasticsearch.common.settings.Setting;
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.common.settings.SettingsFilter;
+import org.elasticsearch.common.util.FeatureFlag;
 import org.elasticsearch.features.NodeFeature;
 import org.elasticsearch.index.reindex.BulkByScrollTask;
 import org.elasticsearch.index.reindex.DeleteByQueryAction;
@@ -44,6 +45,11 @@ public class ReindexPlugin extends Plugin implements ActionPlugin {
 
     public static final ActionType<ListTasksResponse> RETHROTTLE_ACTION = new ActionType<>("cluster:admin/reindex/rethrottle");
 
+    /**
+     * Whether the feature flag to guard the work to make reindex more resilient while it is under development.
+     */
+    static boolean REINDEX_RESILIENCE_ENABLED = new FeatureFlag("reindex_resilience").isEnabled();
+
     @Override
     public List<ActionHandler> getActions() {
         return Arrays.asList(