Add configuration provider and simple provider functions

lib/configuration_provider.dart

@@ -0,0 +1,4 @@
+abstract class ConfigurationProvider {
+  bool containsKey(String key);
+  String? getOrNull(String key, {String? defaultValue});
+}

lib/simple_configuration_provider.dart

@@ -0,0 +1,22 @@
+import 'package:configurable/configuration_provider.dart';
+
+class SimpleConfigurationProvider implements ConfigurationProvider {
+  Map<String, String?> configs = {};
+
+  @override
+  String? getOrNull(String key, {String? defaultValue}) {
+    var value = configs[key];
+
+    if (value == null) {
+      configs[key] = defaultValue;
+      return defaultValue;
+    }
+
+    return value;
+  }
+
+  @override
+  bool containsKey(String key) {
+    return configs.containsKey(key);
+  }
+}

lib/system_config.dart

@@ -0,0 +1,26 @@
+import 'package:configurable/configuration_provider.dart';
+import 'package:configurable/simple_configuration_provider.dart';
+
+class SystemConfig {
+  static ConfigurationProvider? provider;
+
+  static void setProvider(ConfigurationProvider provider) {
+    SystemConfig.provider = provider;
+  }
+
+  static ConfigurationProvider getProvider() {
+    if (provider == null) {
+      setProvider(SimpleConfigurationProvider());
+    }
+
+    return provider!;
+  }
+
+  static String get(String key, {String? defaultValue}) {
+    return getOrNull(key, defaultValue: defaultValue)!;
+  }
+
+  static String? getOrNull(String key, {String? defaultValue}) {
+    return getProvider().getOrNull(key);
+  }
+}