tools: ynl: add a completely generic client

Add a CLI sample which can take in arbitrary request
in JSON format, convert it to Netlink and do the inverse
for output.

It's meant as a development tool primarily and perhaps
for selftests which need to tickle netlink in a special way.

Acked-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>