libsysdecode: use decoder table for PF netlink commands

Introduce a PF netlink command decoder table mapping PFNL commands to
their attribute decoder sets. This replaces the existing switch-based
dispatch and makes it easier to add support for additional PF netlink
commands.

Signed-off-by: Ishan Agrawal <iagrawal9990@gmail.com>
Sponsored-by: Google LLC (GSoC 2026)
Reviewed by: kp