add a gz-omnicopter model (#21818)

* [not-working] add a gz-omnicopter model

* Fix axis directions on omnicopter model

The omnicopter joint axis directions had to be adapted for sdf 1.9 as it has different conventions for joint axis definitions.

* include model from gz-fuel & remove mesh files

* Fix omnicopter model using fuel

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Co-authored-by: Jaeyoung Lim <jalim@ethz.ch>

Tools/simulation/gz/models/omnicopter/model.config

@@ -0,0 +1,15 @@
+<?xml version="1.0"?>
+<model>
+  <name>Omnicopter</name>
+  <version>1.0</version>
+  <sdf version='1.9'>model.sdf</sdf>
+
+  <author>
+   <name>Jaeyoung Lim</name>
+   <email>jalim@ethz.ch</email>
+  </author>
+
+  <description>
+    Omnicopter model for over actuated system
+  </description>
+</model>

Tools/simulation/gz/models/omnicopter/model.sdf

@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<sdf version='1.9'>
+  <include>
+      <name>omnicopter</name>
+      <pose>0 0 0 0 0 0</pose>
+      <uri>https://fuel.gazebosim.org/1.0/PX4/models/Omnicopter</uri>
+    </include>
+</sdf>