add: basic webrtc is working

ui/lib/session_service.dart

@@ -0,0 +1,15 @@
+import 'package:grpc/grpc_or_grpcweb.dart';
+import 'package:ui/gen/signaler_service.pbgrpc.dart';
+
+class SessionService {
+  final SignalerServiceClient _stub;
+  final Map<String, ResponseFuture<AuthToken>> _authTokens = {};
+
+  SessionService(this._stub);
+
+  Future<String> getAuthToken(String cameraID) async {
+    var val = await _authTokens.putIfAbsent(cameraID,
+        () => _stub.createAuthToken(CreateAuthTokenRequest(home: cameraID)));
+    return "Bearer ${val.token}";
+  }
+}