Throttle point cloud topics

Depends on https://github.com/ignitionrobotics/ign-launch/pull/116

Context

Point clouds are found to have large message size, ~5MB for some configurations so added a way for clients to set and limit the rate at which these messages are published over the network

Change

Added a new throttle function to set the publish rate of a topic on the websocket server. By default, it limits point clouds to 1 Hz.