Subscription permission API (#68)

* signal

* request

* implement

* rename event

* ref

* adjustments based on review

* override subscribed

* move events emit

* fix subscription state

* minor change

* update example for permissions demo

* format & subscriptionState as computed property

* prepare macos build

* unsubscribed event when subscription not allowed

lib/src/internal/events.dart

@@ -207,6 +207,19 @@ class SignalSubscribedQualityUpdatedEvent
   });
 }
 
+@internal
+class SignalSubscriptionPermissionUpdateEvent
+    with SignalEvent, EngineEvent, InternalEvent {
+  final String participantSid;
+  final String trackSid;
+  final bool allowed;
+  const SignalSubscriptionPermissionUpdateEvent({
+    required this.participantSid,
+    required this.trackSid,
+    required this.allowed,
+  });
+}
+
 // ----------------------------------------------------------------------
 // Engine events
 // ----------------------------------------------------------------------