Backward compatability when adding fields to an existing msg

[TamarYi]

Bug report

Operating System:
Ubuntu 20.04 64b

Installation type:

Version or commit hash:
5.0.2

DDS implementation:
Fast-RTPS

Steps to reproduce issue:

1. Create a topic with X fields
2. Compile for the publisher
3. Add another field to the topic
4. Compile the subscriber
5. "Listen" to the topic
6. Publish the topic

Expected behavior:
In a different RMW the subscriber receives the full message with the full list of the fields (X+1)
when the last field is set to 0

Actual behavior:
Message is published but the subscriber doesn't receive the message

[clalancette]

We have a draft design PR open on how to do this over in ros-infrastructure/rep#358 . It's going to be a big change, so I'm not sure when we'll get it in. Regardless, I'm going to close this as a duplicate of that in-progress work.