tools/net/ynl: Add --output-json arg to ynl cli

The ynl cli currently emits python pretty printed structures which is
hard to consume. Add a new --output-json argument to emit JSON.

Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Reviewed-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Link: https://lore.kernel.org/r/20240129223458.52046-2-donald.hunter@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>