C2MS12 — Create PSMs for at least XML and REST/JSON

In C2MS 1.0, XSDs were provided by NASA Goddard and published as 'normative' along with the spec. However, these were somewhat incomplete and not maintained by OMG, so these were removed as part of C2MS in 1.1.

The issue is that we do need to supply some PSMs to encourage C2MS usage in the space community. To address GMSEC, it makes sense to supply an XML PSM, but it also seems good to have a REST/JSON PSM to make it more modern and usable in modern architectures. Also consider PSM for Protobuf.

The intent should be to stop using NASA Goddard as the point of contact for all things C2MS. In fact, OMG should supply these PSMs to Goddard as input, rather than Goddard supplying XSDs to OMG as happened in 1.0.

Additionally, a REST/JSON and/or Protobuf PSM should focus on non-pub-sub usage... something that can exist and operate in a services-based architecture. In this case, it could be that the message subjects are ignored as these are used entirely for pub-sub subscriptions/delivery. If that turns out to be the case, we might make clear in the PIM that subjects are optional.

Related to the above, we COULD make subjects part of a PSM rather than PIM???

PSMs can be done as new chapters within the spec, perhaps in a C2MS 1.3 release.