Basic functionality for loading introspection type support libraries

[nnmm]

Given a topic type in the form of a string like "geometry_msgs/msg/Pose", this will

• Parse the string into the package name "geometry_msgs" and the message name "Pose" (see the MessageTypeName type)
• Search the ament index for the prefix of the "geometry_msgs" package. In the prefix, the introspection type support library will be in lib/libgeometry_msgs__rosidl_typesupport_introspection_c.so (via the ament_rs crate)
• Load that library using libloading (see get_type_support_library())
• Extract the symbol corresponding to the "Pose" type from that library (see get_type_support_handle())

This commit intentionally leaves out an important part of the DynamicMessageMetadata type: The representation of the message structure. This will follow in another commit.

[esteve]

@nnmm have you looked into https://github.com/PlotJuggler/plotjuggler-ros-plugins/tree/development/plugins/ros2_introspection? This may solve a similar problem and/or serve as inspiration.

[nnmm]

@esteve Thanks for the link, I didn't know that one yet. I think my branch already covers the functionality from that package and more.

[jhdcs]

Sorry for the wait, this seems good to me! I don't see any glaring issues, at any rate.

[nnmm]

Thank you very much @jhdcs!

@esteve, do you have any further comments? Otherwise I'll go ahead and merge this.

[nnmm]

@jhdcs Could you reapprove? I had to rebase onto main.

[nnmm]

Thank you!