Expose provider::conflict_gtid() method for GRP

include/wsrep/provider.hpp

@@ -470,6 +470,14 @@ namespace wsrep
          */
         virtual void* native() const = 0;
 
+        /**
+         * Get conflicting gtid for the given transaction handle, if any.
+         */
+        virtual wsrep::gtid conflict_gtid(const wsrep::ws_handle&) const
+        {
+            return wsrep::gtid::undefined();
+        }
+
         /**
          * Services argument passed to make_provider. This struct contains
          * optional services which are passed to the provider.