Interface for a function that returns the serialized size of a message

[kfabian]

Feature request

Feature description

I noticed that none of the major rmw implementations implements rmw_get_serialized_message_size. I wonder if that is the case because it works on the rosidl_runtime_c__Sequence__bound struct and not an actual instance of a message. I could not really grasp how the rosidl_runtime_c__Sequence__bound should be used and to what structure * data should be pointing to.

Implementation considerations

Maybe it would be helpful to add a function interface that works on an actual instance of a message, that should be more straightforward to implement, e.g. for cyclonedds similar to this code.

[clalancette]

Hm, the fact that none of them implement it means the API should probably be deprecated and removed. I think this was part of an early attempt to be able to send preallocated messages, but never got implemented. @mjcarroll can you comment here?

[mjcarroll]

I think that is the origin of it. I think that the loaned message APIs came after and superceded this concept. I think we can safely deprecate and remove the existing APIS.

Second, I think that the idea of working on a message instance may work as well. The Sequence__bound structure was intended to provide bounds for unbounded sequence fields so that allocation size could be correctly computed.