selftests: drv-net: add PSP responder

PSP tests need the remote system to support PSP, and some PSP capable
application to exchange data with. Create a simple PSP responder app
which we can build and deploy to the remote host. The tests themselves
can be written in Python but for ease of deploying the responder is in C
(using C YNL).

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Daniel Zahka <daniel.zahka@gmail.com>
Link: https://patch.msgid.link/20250927225420.1443468-4-kuba@kernel.org


Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>