Launch gz_spawn_model from xml

[caguero]

🎉 New feature

Part of https://github.com/gazebosim/ros_gz/issues/552

Summary

This patch lets you spawn a gz sim model from an XML file.

How to test it?

Launch gz sim:

ros2 launch ros_gz_sim gz_sim.launch.py gz_args:=empty.sdf

Hit play.

Use the provided gz_spawn_model.launch to spawn a vehicle (change file accordingly):

ros2 launch ros_gz_sim gz_spawn_model.launch world:=empty file:=/home/caguero/ros_gz_ws/src/ros_gz/ros_gz_sim_demos/models/vehicle/model.sdf name:=my_vehicle x:=5.0 y:=5.0 z:=0.5

A vehicle should be loaded into the scene.

Confirm that no container has been created:

caguero@cold:~/ros_gz_ws$ ros2 component list

Test it

Checklist

• [x] Signed all commits for DCO
• [ ] Added tests
• [ ] Added example and/or tutorial
• [ ] Updated documentation (as needed)
• [ ] Updated migration guide (as needed)
• [ ] Consider updating Python bindings (if the library has them)
• [ ] codecheck passed (See contributing)
• [ ] All tests passed (See test coverage)
• [ ] While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.