feat: reorganize attachments provider and model structure

lib/dashbot/core/model/chat_attachment.dart

@@ -0,0 +1,19 @@
+import 'package:flutter/services.dart';
+
+class ChatAttachment {
+  final String id;
+  final String name;
+  final String mimeType;
+  final int sizeBytes;
+  final Uint8List data;
+  final DateTime createdAt;
+
+  ChatAttachment({
+    required this.id,
+    required this.name,
+    required this.mimeType,
+    required this.sizeBytes,
+    required this.data,
+    required this.createdAt,
+  });
+}