add support for UAConfiguration registrar_server (#397)

- https://github.com/versatica/JsSIP/blob/004d160968ec71982beca46d021e3cab746ac55d/lib/UA.d.ts#L43C3-L43C19

Author: VictorUvarov

lib/src/sip_ua_helper.dart

@@ -142,6 +142,7 @@ class SIPUAHelper extends EventManager {
     _settings.session_timers_refresh_method =
         uaSettings.sessionTimersRefreshMethod;
     _settings.instance_id = uaSettings.instanceId;
+    _settings.registrar_server = uaSettings.registrarServer;
 
     try {
       _ua = UA(_settings);
@@ -711,6 +712,7 @@ class UaSettings {
   String? ha1;
   String? displayName;
   String? instanceId;
+  String? registrarServer;
 
   /// DTMF mode, in band (rfc2833) or out of band (sip info)
   DtmfMode dtmfMode = DtmfMode.INFO;