Use Zenoh in gz-transport

[azeey]

gz-transport has its own implementation of discovery and message transport built on top of zeromq. This has served Gazebo well for a long time, but it's had its downsides:

• Having our own custom discovery protocol means
  • it is difficult to support languages other than C++
  • we have to maintain it
• gz-transport uses Multicast UDP for discovery which doesn't work out of the box on some platforms due to firewall settings
• gz-transport has one thread to service messages which can be a performance bottleneck.

Adopting Zenoh would solve some or all of these problems. The ROS 2 project has invested a lot on Zenoh and Gazebo also using Zenoh could create better Gazebo/ROS integration down the road.