feat: extension support (#78)

ui/flutter/lib/api/model/install_extension.dart

@@ -0,0 +1,18 @@
+import 'package:json_annotation/json_annotation.dart';
+
+part 'install_extension.g.dart';
+
+@JsonSerializable()
+class InstallExtension {
+  bool devMode;
+  String url;
+
+  InstallExtension({
+    this.devMode = false,
+    required this.url,
+  });
+
+  factory InstallExtension.fromJson(Map<String, dynamic> json) =>
+      _$InstallExtensionFromJson(json);
+  Map<String, dynamic> toJson() => _$InstallExtensionToJson(this);
+}