C2MS13 — Consider adding custom fields at the definition of the C2MS Message level

All C2MS Messages can be augmented via CUSTOM-FIELDS, but this is at an individual message. Consider if we should allow the C2MS Message Type to define CUSTOM-FIELDS, so that all messages in a given domain would have this CUSTOM-FIELD.

One note: This is different from C2MS13-62 which was to consider doing this at the Envelope level, which the RTF decided against.

Another note: Just capturing one reason not to do this. Adding a custom field capability at the C2MS Message level that would then be inherited by all subclasses would mean that the parent class would no longer be a C2MS Message, but a derivative of it, which is redefining the base class of all messages. We've tried to stay away from that in the past, and see the related issue regarding message envelope. This present issue may have the same concern.