C2MS11 — PROD message, add fields PROD_SUBTYPE

There are n+2 number of optional elements between ME4 and ME7. These fields map the field(s) PROD-SUBTYPE.n.NAME.
API Workaround: API only maps the first two of these product subtypes, and the user will have set subject if they want more.
Required C2MS Spec changes: Ditch the n+2 logic in the middle of a message subject and only display the first two PROD-SUBTYPE values in the subject (or FILL if not used). A subject that can have dynamically shifting positions of its subject elements has serious implications on compatibility.

*PROD and SUBTYPE should be enough but if not... *

We lack use cases that show and problem here. This is just a possible problem for what seems like the unlikely case of product with a subtype that isn't enough to differentiate it from another one, and that then the 2 additional ME fields wouldn't be sufficient.

Overall with 4 pre-canned ME required and optional fields for a subject, it's difficult to see that this is not enough to differentiate some imaginary product in a topic/subject.

In fact due the complexity of such a product I kind of think it's better to just let such a need if it ever exists to just find some other localized way to handle it.