feat(xep): Allow changing the SASL2 user agent

integration_tests/test/sasl2_test.dart

@@ -44,13 +44,12 @@ void main() {
       FASTSaslNegotiator(),
       Bind2Negotiator(),
       StartTlsNegotiator(),
-      Sasl2Negotiator(
-        userAgent: const UserAgent(
+      Sasl2Negotiator()
+        ..userAgent = const UserAgent(
           id: 'd4565fa7-4d72-4749-b3d3-740edbf87770',
           software: 'moxxmpp',
           device: "PapaTutuWawa's awesome device",
         ),
-      ),
     ]);
 
     final result = await conn.connect(