Refactor genai package to new modular interface

Reorganized the genai package by removing legacy LLM-related files and introducing a new modular interface under the 'interface' directory. Added provider-specific model classes, centralized constants, and updated the example to use the new API and data structures. Updated exports in genai.dart and improved dependency management.

packages/genai/lib/models/model_config.dart

@@ -0,0 +1,74 @@
+import 'model_config_value.dart';
+
+class ModelConfig {
+  final String id;
+  final String name;
+  final String description;
+  final ConfigType type;
+  final ConfigValue value;
+
+  ModelConfig({
+    required this.id,
+    required this.name,
+    required this.description,
+    required this.type,
+    required this.value,
+  }) {
+    assert(checkTypeValue(type, value));
+  }
+
+  ModelConfig updateValue(ConfigValue value) {
+    return ModelConfig(
+      id: id,
+      name: name,
+      description: description,
+      type: type,
+      value: value,
+    );
+  }
+
+  factory ModelConfig.fromJson(Map x) {
+    final id = x['id'] as String?;
+    final name = x['name'] as String?;
+    final description = x['description'] as String?;
+    final type = x['type'] as String?;
+    final value = x['value'] as String?;
+
+    final cT = getConfigTypeEnum(type);
+    final cV = deserilizeValue(cT, value);
+
+    return ModelConfig(
+      id: id ?? "",
+      name: name ?? "",
+      description: description ?? "",
+      type: cT,
+      value: cV,
+    );
+  }
+
+  Map<String, String?> toJson() {
+    return {
+      'id': id,
+      'name': name,
+      'description': description,
+      'type': type.name.toString(),
+      'value': value.serialize(),
+    };
+  }
+
+  ModelConfig copyWith({
+    String? id,
+    String? name,
+    String? description,
+    ConfigType? type,
+    ConfigValue? value,
+  }) {
+    return ModelConfig(
+      id: id ?? this.id,
+      name: name ?? this.name,
+      description: description ?? this.description,
+      type: type ?? this.type,
+      value: value ?? this.value,
+    );
+  }
+}