Update sip_ua.dart (#456)

* Update sip_ua.dart

Added export 'src/sip_message.dart';

* import sorter.

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Co-authored-by: [email protected] <[email protected]>

Author: NicolasGodfather

lib/sip_ua.dart

@@ -1,3 +1,4 @@
 export 'src/enum_helper.dart';
+export 'src/sip_message.dart';
 export 'src/sip_ua_helper.dart';
 export 'src/transport_type.dart';