Add support for starting and stopping the tunnel

This commit introduces the Tunnel view model object which has
support for asking for permission, starting and stopping the
tunnel. It automatically updates its state and publishes
changes as an ObservableObject.

Apple/App/TunnelView.swift

@@ -0,0 +1,37 @@
+import SwiftUI
+
+struct TunnelView: View {
+
+    @ObservedObject
+    var tunnel: Tunnel
+
+    var body: some View {
+        VStack {
+            Text(verbatim: tunnel.status.description)
+            switch tunnel.status {
+            case .connected(_):
+                Button("Disconnect", action: stop)
+            case .permissionRequired:
+                Button("Allow", action: configure)
+            case .disconnected:
+                Button("Start", action: start)
+            default:
+                EmptyView()
+            }
+        }
+        .task { await tunnel.update() }
+        .padding()
+    }
+
+    private func start() {
+        try? tunnel.start()
+    }
+
+    private func stop() {
+        tunnel.stop()
+    }
+
+    private func configure() {
+        Task { try await tunnel.configure() }
+    }
+}