registerPushToken method
PUT /v1/sessions/current/push-token: lets the runtime reach this
device with app.notify events while the app is in the background.
Implementation
@override
Future<void> registerPushToken(
KletsoSession session,
KletsoPushToken token,
) async {
await _latency();
final s = _auth(session);
_pushTokens[s.endUserId] = token;
}