net/sfc: add representors proxy infrastructure

net/sfc: add representors proxy infrastructure

Representor proxy is a mediator between virtual functions and port
representors. It forwards traffic between virtual functions and port
representors performing base PF ethdev + VF's representor traffic
(de-)multiplexing. The implementation will be provided by later patches.

Signed-off-by: Igor Romanov <igor.romanov@oktetlabs.ru>
Signed-off-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
Reviewed-by: Andy Moreton <amoreton@xilinx.com>
Reviewed-by: Ivan Malov <ivan.malov@oktetlabs.ru>