gridappsd-field-bus-lib duplicates the stomp arg shape branch; share _unpack_stomp_args

[craigpnnl]

The sibling package gridappsd-field-bus-lib (for example field_proxy_forwarder.py) contains its own duplicate stomp argument shape branch, the same bug class fixed in gridappsd-python-lib's goss.py in #207.

Once _unpack_stomp_args lands in gridappsd-python-lib, expose it as a shared utility that the field bus package imports rather than re deriving the branch. This prevents the same stomp 6 versus stomp 8 AttributeError from recurring there.