Task: Add async support and functions for configurable and add tests for async testing and updated the functions with default

test/config_async_test.dart

@@ -0,0 +1,70 @@
+import 'package:configurable/configurable.dart'
+    show
+        getConfigOrNullAsync,
+        hasConfigkeyAsync,
+        setConfigAsync,
+        removeConfigAsync;
+import 'package:configurable/simple_configuration_provider.dart';
+import 'package:configurable/system_config.dart';
+import 'package:test/test.dart';
+
+void main() {
+  test('get system config by key', () async {
+    var key = 'app.name';
+    var value = 'CUBETIQ';
+
+    // set simple provider
+    SystemConfig.setProvider(SimpleConfigurationProvider());
+
+    var result = await SystemConfig.getOrNullAsync(key, defaultValue: value);
+
+    expect(value, equals(result));
+    expect(value, equals(await SystemConfig.getOrNullAsync(key)));
+  });
+
+  test('get config by key with function', () async {
+    var key = 'app.name';
+    var value = 'CUBETIQ Solution';
+
+    // set simple provider
+    SystemConfig.setProvider(SimpleConfigurationProvider());
+
+    var result = await getConfigOrNullAsync(key, defaultValue: value);
+
+    expect(value, equals(result));
+    expect(true, equals(await hasConfigkeyAsync(key)));
+    expect(value, equals(await getConfigOrNullAsync(key)));
+  });
+
+  test('set config by key and value with function', () async {
+    var key = 'app.name';
+    var value = 'CUBETIQ Solution';
+
+    // set simple provider
+    SystemConfig.setProvider(SimpleConfigurationProvider());
+
+    await setConfigAsync(key, value);
+
+    var result = await getConfigOrNullAsync(key);
+
+    expect(value, equals(result));
+  });
+
+  test('set and then remove config by key and value with function', () async {
+    var key = 'app.name';
+    var value = 'CUBETIQ Solution';
+
+    // set simple provider
+    SystemConfig.setProvider(SimpleConfigurationProvider());
+
+    await setConfigAsync(key, value);
+
+    var result = await getConfigOrNullAsync(key);
+
+    expect(value, equals(result));
+
+    await removeConfigAsync(key);
+
+    expect(null, equals(await getConfigOrNullAsync(key)));
+  });
+}