DDSIRTP26 — Require extended header to be the first submessage

If the RTPS_HE submessage is present, can we impose a restriction that requires it to be the first submessage of the RTPS packet? Placing it anywhere else renders some of its fields less useful (or useless). It also makes handling things like checksums trickier, as it requires you to:
1. Iterate over all submessages to determine if the RTPS_HE is present,
2. Verify the checksum (if present),
3. Go back to start, iterate over all submessages again and now process their contents, since we have now validated the checksum.

Other fields like the length of message also becomes less useful if it's the last submessage.