openflow_input: add bsn_egr_port_group_id extension
java_gen: support of_oxm_bsn_egr_port_group_id

It's basically a class-id, so using that type.
Merge into master from pull request #222:
openflow_input: add bsn_egr_port_group_id extension (https://github.com/floodlight/loxigen/pull/222)