openonuAdapterGo: OMCI extended message set - support ONU image status retrieval

If the OMCC version of a BBSIM ONU is configured to a value that supports the OMCI extended message set (see VOL-4678), OMCI get requests for ONU image status retrieval in extended message format shall be answered with the corresponding OMCI responses in extended message format. It is assumed that the number of necessary ONU image status retrieval messages is significantly reduced due to the greater length of the message content.