feat: add sendDtmf method to send DTMF tones during a call

ios/Classes/LiblinphoneFlutterPlugin.swift

@@ -209,7 +209,21 @@ public class LiblinphoneFlutterPlugin: NSObject, FlutterPlugin {
         case "syncCurrentState" :
             linphoneBridge.syncCurrentState()
             result(true)
+
+        case "sendDtmf":
+            guard let args = call.arguments as? [String: Any],
+                  let tone = args["tone"] as? String else {
+                result(FlutterError(
+                    code: "INVALID_ARGUMENTS",
+                    message: "Missing required arguments",
+                    details: nil
+                ))
+                return
+            }
             
+            let success = linphoneBridge.sendDtmf(tone: tone)
+            result(success)
+
         default:
             result(FlutterMethodNotImplemented)
         }