Restore marker support

[caguero]

Fixes #596.

This PR restores the support to overlay a 3D marker on the scene.

Known limitations:

• Currently, it's not possible to render a TEXT marker. See this open issue.
• It's not possible to pass a text material like in Gazebo classic (Gazebo/Red). Instead, it's possible to set ambient and diffuse values. I decided not to expose it as it seems that we're always using the same color for the markers. Let me know otherwise and we could expose it.

How to test it?

Launch one of the stationkeeping/wayfinding world and check that a white marker shows up. Example:

ros2 launch vrx_gz competition.launch.py world:=practice_2022_stationkeeping2_task

[M1chaelM]

Thanks! This is great. The color definitely does not matter. The text was sometimes useful for debugging new course designs for wayfinding, but I think we can live without it.