Rethink shape_msgs/SolidPrimitive.msg

The current version of SolidPrimitive.msg looks like: https://github.com/ros2/common_interfaces/blob/b7e2ad503e1648468b1dc766c43aac12cc9aee5a/shape_msgs/msg/SolidPrimitive.msg .

If you look closely at that message, though, it has two problems:

1. The messages share very little infrastructure with each other.
2. We don't provide the facility to generate a shape with holes in it.

The first problem suggests that we should actually have 5 different messages: SolidBox.msg, SolidSphere.msg, SolidCylinder.msg, SolidCone.msg, and SolidPrism.msg . That would make these messages a lot more semantically defined, rather than artificially stuffing them into the same message.

The second problem is debatable about whether we want to support it. Gazebo, for instance, supports something called "poly lines" which are shapes with holes in them. An Example of this is in https://classic.gazebosim.org/tutorials?tut=extrude_svg&cat=model_editor_top#CreateaGazeboModel . Whether those shapes could be considered "primitive" is an open question, but one we should consider allowing. If we did allow this, we would need to think through the implications of this and how we present it to the user.

@tfoote @chapulina FYI.

Start by reading shape_msgs/msg/SolidPrimitive.msg and the linked Gazebo poly-lines example. Work through whether the proposed SolidBox.msg, SolidSphere.msg, SolidCylinder.msg, SolidCone.msg, and SolidPrism.msg split is appropriate, and whether shapes with holes belong in these interfaces. Done means the open design questions have a decided interface direction.