Add ParamVec and bridge from Ignition

[mjcarroll]

🎉 New feature

• Follow up to Add support for converting Any <-> ParamValue #260
• All of the support needed for Add RF range sensor osrf/mbzirc#151

Summary

• Introduces ros_ign_interfaces::msg::ParamVec for storing a list of Parameters that are int, bool, double, or string.
• Introduces bridge for ignition::msgs::param to ros_ign_interfaces::msg::ParamVec
• Introduces bridge for ignition::msgs::param_v to ros_ign_interfaces::msg::ParamVec
• Adds specific unit tests for the conversion process, as there are some caveats.

Caveats: Ignition allows for parameter messages to be infinitely nested. ROS stores it's parameters in a flat list. When there are nested messages in Ignition, they will be prefixed with child_<N>/ in their parameter name to represent where in the structure they came from. This process is not applied in reverse.

If the source is an ignition::msgs::param_v, it will be flattened to be stored in the ros structure. This flattening happens by prefixing each of the entries with param_<N>/

Test it

• Unit tests covering the conversion have been added.
• Integrations tests have been added.

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• 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

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[iche033]

Changes look good to me. As for the todo on converting ROS ParamVec back to IGN Param_V, I think maybe we should do this soon, e.g. before the next Galactic release, so that people won't depend on the current behavior.