DDSIRTP21 — Clarify when a Payload is present in a submessage

Some submessages can optionally include a payload. For example the DATA submessage includes a payload in most cases, except in some special cases where the submessage indicates unregistration and/or disposal of the data.

Currently the presence of the payload is tied to the value of the unregister and dispose flags. This is inflexible and also does not support some use-cases where it is desirable to send data with dispose messages. It would be much better to make the presence of the Data Payload separately configurable in the submessage.

Resolution:
Introduce a new submessage element that represents a general submessage payload whose type is specified by submessage meta-information

Introduce a new submessage element that represents a general submessage payload whose type is specified by submessage meta-information