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

example/lib/exts.dart

@@ -135,4 +135,23 @@ extension LKExampleExt on BuildContext {
           ],
         ),
       );
+
+  Future<bool?> showSubscribePermissionDialog() => showDialog<bool>(
+        context: this,
+        builder: (ctx) => AlertDialog(
+          title: const Text('Allow subscription'),
+          content: const Text(
+              'Allow all participants to subscribe tracks published by local participant?'),
+          actions: [
+            TextButton(
+              onPressed: () => Navigator.pop(ctx, false),
+              child: const Text('NO'),
+            ),
+            TextButton(
+              onPressed: () => Navigator.pop(ctx, true),
+              child: const Text('YES'),
+            ),
+          ],
+        ),
+      );
 }