Add ROS Package Paths to Gazebo Resource Paths

[DLu]

I've had it with meshes not loading when I spawn URDF into Gazebo.

This PR aims to eventually

• Extract all the ROS package names from mesh filenames of the form package://package_name/whatever
• Determine which paths aren't on the Gazebo Resource Path and add as necessary, using server service calls

However, I'm still learning to do calls to gazebo services, so that part is a WIP and I would appreciate assistance with.

(Note: I've developed this code with Foxy but the PR is for the ros2 branch)

[DLu]

Superceded by https://github.com/ros-simulation/gazebo_ros_pkgs/issues/1500