Support particle emitters via websockets

Related tasks

• gz3d!44 (merged)
• https://github.com/ignitionrobotics/ign-launch/pull/104
• https://github.com/ignitionrobotics/ign-gazebo/pull/730
• https://github.com/osrf/sdformat/pull/528

Context

This adds support for getting particle emitters from the websocket server and using gz3d to visualize the emitters.