tools: ynl: decode hex input

tools: ynl: decode hex input

This patch adds support for decoding hex input, so
that binary attributes can be read through --json.

Example (using future wireguard.yaml):
 $ sudo ./tools/net/ynl/pyynl/cli.py --family wireguard \
   --do set-device --json '{"ifindex":3,
     "private-key":"2a ae 6c 35 c9 4f cf <... to 32 bytes>"}'

In order to somewhat mirror what is done in _formatted_string(),
then for non-binary attributes attempt to convert it to an int.

Signed-off-by: Asbjørn Sloth Tønnesen <ast@fiberby.net>
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://patch.msgid.link/20250915144301.725949-11-ast@fiberby.net
Signed-off-by: Jakub Kicinski <kuba@kernel.org>