Add support for non-POD types

[nightduck]

For objects that link to other objects, first store a string detailing the class name of the object in a base memory allocation. That way receiving subscribers can verify type-match between publisher and subscriber.
Next follows a list of pointers to any non-static members. These pointers can be null.

In rmw_hazcat, this format will be translated into a proper rosidl interpretation and vice versa. Can this be done with by maintaining zero-copy and just rearranging pointers? The class name will be the message type (eg std_msgs/PoseStamped).