Reshard security authz support (elastic#139282)

This commit ensures that the system context can run reshard split
actions.

Author: Tim-Brooks

x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/security/authz/privilege/SystemPrivilege.java

@@ -36,6 +36,7 @@ public final class SystemPrivilege extends Privilege {
         "indices:admin/template/put", // needed for the TemplateUpgradeService
         "indices:admin/template/delete", // needed for the TemplateUpgradeService
         "indices:admin/seq_no/global_checkpoint_sync*", // needed for global checkpoint syncs
+        "indices:admin/reshard/split*", // needed for reshard operations
         RetentionLeaseSyncAction.ACTION_NAME + "*", // needed for retention lease syncs
         RetentionLeaseBackgroundSyncAction.ACTION_NAME + "*", // needed for background retention lease syncs
         RetentionLeaseActions.ADD.name() + "*", // needed for CCR to add retention leases