selftests: openvswitch: add psample action

Add sample and psample action support to ovs-dpctl.py.

Refactor common attribute parsing logic into an external function.

Reviewed-by: Aaron Conole <aconole@redhat.com>
Signed-off-by: Adrian Moreno <amorenoz@redhat.com>
Link: https://patch.msgid.link/20240704085710.353845-8-amorenoz@redhat.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>