BBSIM: OMCI extended message set - support get requests

This story is a precondition to support implementation of VOL-4688.
Current BBSIM implementation aready sends OMCC version 0xB4 in ME Onu2g which means that the ONU supports OMCI extended message set. But a later test request to get Onu2g-data in extended message format fails at BBSIM-side already during parsing in OLT-part because each message is treated as a baseline one. By this story an MVP implementation has to be done to support get requests in extended format.