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

example/example_flutter_secure_storage_provider.dart

@@ -0,0 +1,90 @@
+import 'package:flutter_secure_storage/flutter_secure_storage.dart';
+import 'package:configurable/mutable_configurable_provider.dart';
+
+class LocalStorageProvider
+    extends MutableConfigurationProvider  {
+  static FlutterSecureStorage _storage;
+
+  FlutterSecureStorage _getStorage() {
+    if (_storage == null) {
+      _storage = new FlutterSecureStorage();
+    }
+
+    return _storage;
+  }
+
+  @override
+  bool containsKey(String key) {
+    var _hasKey = false;
+    _getStorage().containsKey(key: key).then((value) => _hasKey = value);
+
+    return _hasKey;
+  }
+
+  @override
+  Future<bool> containsKeyAsync(String key) async {
+    return await _getStorage().containsKey(key: key);
+  }
+
+  @override
+  String? getOrNull(String key, {String defaultValue}) {
+    String _value;
+    _getStorage().read(key: key).then((value) => {
+      _value = value
+    });
+
+    if (_value == null && defaultValue != null) {
+      return defaultValue;
+    } else {
+      return _value;
+    }
+  }
+
+  @override
+  Future<String?> getOrNullAsync(String key, {String defaultValue}) async {
+    String _value = await _getStorage().read(key: key);
+
+    if (_value == null && defaultValue != null) {
+      return defaultValue;
+    } else {
+      return _value;
+    }
+  }
+
+  @override
+  void remove(String key) {
+    _getStorage().delete(key: key);
+  }
+
+  @override
+  Future<void> removeAsync(String key) async {
+    await _getStorage().delete(key: key);
+  }
+
+  @override
+  void removeAll() {
+    _getStorage().deleteAll();
+  }
+
+  @override
+  Future<void> removeAllAsync() async {
+    await _getStorage().deleteAll();
+  }
+
+  @override
+  void set(String key, String value) {
+    _getStorage().write(key: key, value: value);
+  }
+
+  @override
+  Future<void> setAsync(String key, String value) async {
+    await _getStorage().write(key: key, value: value);
+  }
+
+  @override
+  Future<void> setAllAsync(Map<String, String> configs) async {
+    configs.entries.forEach((element) {
+      setAsync(element.key, element.value);
+    });
+  }
+}