UDP schema should be designed to allow another schema to define the payload

[mbeckerle]

UDP datagrams carry a chunk of data.

We'd like to be able to use the UDP schema, but redefine (earlier on the classpath), the payload of the UDP packet, so that it can be parsed.

This trick is used by the mil-std-2045 schema to compute the message size. UDP should do the same thing so that one can supply a DFDL schema for the UDP packet payload, and have that parsed also.