libsysdecode: add Netlink attribute decoding infrastructure

Introduce a generic Netlink attribute decoding framework based on
attribute decoder tables. The framework supports decoding primitive
attribute types as well as nested attributes and can be reused by
different Generic Netlink families.

Signed-off-by: Ishan Agrawal <iagrawal9990@gmail.com>
Sponsored-by: Google LLC (GSoC 2026)
Reviewed-by: kp
Pull-Request: https://github.com/freebsd/freebsd-src/pull/2337