tools/net/ynl: Add nest-type-value decoding

The nlctrl genetlink-legacy family uses nest-type-value encoding as
described in Documentation/userspace-api/netlink/genetlink-legacy.rst

Add nest-type-value decoding to ynl.

Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://lore.kernel.org/r/20240306231046.97158-5-donald.hunter@gmail.com


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