DIAMETER: Split Gy and Gx messages to their own TS files

Those interfaces reuse messages from other Diameter RFCs, but changing
the contents of the messages, eg some Mandatory fields like
Service-Context-Id which are mandatory in RFC5006, doesn't even appear
in Gx CCR in TS 29.212.

Keeping them well separated helps in avoiding confusion on users fo the
messages.

Change-Id: Ibe0d5f263813d5083e020c942283f214983162b4
7 files changed