Support parameter updates on nodes that don't have /parameter_events

[clalancette]

When constructing a ROS 2 node, it is possible to disable the /parameter_events topic. Right now, ros_network_viz depends on /parameter_events to get parameter updates in the network. For nodes that do not have /parameter_events, we should instead poll the get_parameters service periodically to check for parameter updates. Note that it is also possible for a ROS 2 node to be constructed without those services, so there is no way we can always guarantee that we can show parameters.