feat(ui): add the ability to add a background image in MessageListView

packages/stream_chat_flutter/example/lib/tutorial_part_6.dart

@@ -63,6 +63,12 @@ class MyApp extends StatelessWidget {
           borderRadius: BorderRadius.circular(8),
         ),
       ),
+      messageListViewTheme: const MessageListViewThemeData(
+          backgroundColor: Colors.grey,
+          backgroundImage: DecorationImage(
+            image: AssetImage('assets/background_doodle.png'),
+            fit: BoxFit.cover,
+          )),
       otherMessageTheme: MessageThemeData(
         messageBackgroundColor: colorTheme.textHighEmphasis,
         messageTextStyle: TextStyle(

packages/stream_chat_flutter/example/pubspec.yaml

@@ -47,8 +47,8 @@ flutter:
   uses-material-design: true
 
   # To add assets to your application, add an assets section, like this:
-  # assets:
-  #   - images/a_dot_burr.jpeg
+  assets:
+    - assets/background_doodle.png
   #   - images/a_dot_ham.jpeg
 
   # An image asset can refer to one or more resolution-specific "variants", see