C2MS12 — Allow custom message subtypes

C2MS12-53 calls for a way to make C2MS-defined messages extensible. This issue is to allow new TYPES of messages to be defined, within the construct of C2MS. This is to avoid having new messages defined that are not really C2MS Messages.

This could also alleviate C2MS12-22.

If we do this, we should consider allowing the user to specify a MESSAGE-SUBTYPE that is custom (though the MESSAGE-TYPE would still be just MSG/REQ/RESP). A couple of ways this could be done would be to say it must always start with "CUSTOM-", such as "CUSTOM-MISSIONX-CALCULATE-ORBIT". Note that if we do this, we need to change the Description in Table 8-2 for MESSAGE-SUBTYPE.

Alternatively, we could use "CUSTOM" as the MESSAGE-SUBTYPE, and use ME1 to designate the "Custom Message Subtype". In many ways, this is cleaner, but doesn't take advantage of SUBTYP being a part of the message subject directly and uses up one of the ME slots.

Either way, we would describe that they should use naming that avoids conflicts as we said in the resolution for C2MS12-53.

We should consider some kind of registry, perhaps... maybe maintained at the DOMAIN1 level.

In C2MS 2.0, we could consider adding a SubSpecification to go along with the Specication (C2MS), but that wouldn't be possible in 1.x.

If we do anyting in 1.x, we should add a clear descriptoin of how it works, perhaps near or in 6.2.3.4 Message Elements of the C2MS Subject Name.