Use low poly mesh / primitive shapes in gimbal collisions

[srmainwaring]

Feature request

Issue

The gimbal models use the visual meshes for collisions. This causes significant performance slowdown in complex worlds.

For example: https://discuss.ardupilot.org/t/gazebo-harmonic-slow-real-time-factor-on-baylands-world/125284/1

Suggested solution

Replace the visual meshes used in the gimbal collisions with one or more primitive shapes.

Platform
[ x ] All
[ ] AntennaTracker
[ ] Copter
[ ] Plane
[ ] Rover
[ ] Submarine