Implemented StreamAttachmentPackage and fixed operations in widgets linked to full screen implementation, also fixed tests

packages/stream_chat_flutter/lib/src/stream_attachment_package.dart

@@ -0,0 +1,18 @@
+import 'package:stream_chat_flutter/stream_chat_flutter.dart';
+
+/// The [StreamAttachmentPackage] class is basically meant to wrap
+/// individual attachments with their corresponding message
+class StreamAttachmentPackage {
+  /// Default constructor to prepare an [StreamAttachmentPackage] object
+  StreamAttachmentPackage({
+    required this.attachment,
+    required this.message,
+  });
+
+  /// This is the individual attachment
+  final Attachment attachment;
+
+  /// This is the message that the attachment belongs to
+  /// The message object may have attachemnt(s) other than the one packaged
+  final Message message;
+}