added lazy query support

fixed possible race condition of simultanous refetches of the same query

packages/example/lib/lazy_query.dart

@@ -0,0 +1,36 @@
+import 'package:fl_query/fl_query.dart';
+import 'package:flutter/material.dart';
+
+final lazyQueryJob = QueryJob<String, String>(
+    queryKey: "non_enabled_query",
+    enabled: false,
+    task: (queryKey, data) {
+      return Future.delayed(const Duration(milliseconds: 500),
+          () => "Hello from $queryKey with $data");
+    });
+
+class LazyQuery extends StatelessWidget {
+  const LazyQuery({Key? key}) : super(key: key);
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(),
+      body: QueryBuilder<String, String>(
+        job: lazyQueryJob,
+        externalData: "Love",
+        builder: (context, query) {
+          return Column(
+            children: [
+              Text("Query Data::: ${query.data ?? "Loading"}"),
+              ElevatedButton(
+                child: const Text("Fetch Query"),
+                onPressed: () => query.refetch(),
+              ),
+            ],
+          );
+        },
+      ),
+    );
+  }
+}