eventdev: add eth Tx adapter APIs

eventdev: add eth Tx adapter APIs

The ethernet Tx adapter abstracts the transmit stage of an
event driven packet processing application. The transmit
stage may be implemented with eventdev PMD support or use a
rte_service function implemented in the adapter. These APIs
provide a common configuration and control interface and
an transmit API for the eventdev PMD implementation.

The transmit port is specified using mbuf::port. The transmit
queue is specified using the rte_event_eth_tx_adapter_txq_set()
function.

Signed-off-by: Nikhil Rao <nikhil.rao@intel.com>
Acked-by: Jerin Jacob <jerin.jacob@caviumnetworks.com>