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 <[e-mail]>
Reviewed-by: Donald Hunter <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

tools/net/ynl/pyynl/lib/ynl.py

···
                 raw = ip.packed
             else:
                 raw = int(ip)
+        elif attr_spec.display_hint == 'hex':
+            if attr_spec['type'] == 'binary':
+                raw = bytes.fromhex(string)
+            else:
+                raw = int(string, 16)
         else:
             raise Exception(f"Display hint '{attr_spec.display_hint}' not implemented"
                             f" when parsing '{attr_spec['name']}'")