implement changing username

flutter/lib/api/api_client.dart

@@ -162,6 +162,20 @@ class APIClient {
     );
   }
 
+  static Future<User> updateUser(TokenSource auth, String uid, {String? username, String? proToken}) async {
+    return await _request(
+      name: 'updateUser',
+      method: 'PATCH',
+      relURL: 'users/$uid',
+      jsonBody: {
+        if (username != null) 'username': username,
+        if (proToken != null) 'pro_token': proToken,
+      },
+      fn: User.fromJson,
+      authToken: auth.getToken(),
+    );
+  }
+
   static Future<Client> addClient(TokenSource auth, String fcmToken, String agentModel, String agentVersion, String? name, String clientType) async {
     return await _request(
       name: 'addClient',