Add a test to verify sampling packets via psample works.

In order to do that, create a subcommand in ovs-dpctl.py to listen to
on the psample multicast group and print samples.
Reviewed-by: Aaron Conole <aconole@redhat.com>
Tested-by: Ilya Maximets <i.maximets@ovn.org>
Signed-off-by: Adrian Moreno <amorenoz@redhat.com>
Link: https://patch.msgid.link/20240704085710.353845-11-amorenoz@redhat.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>