tools/net/ynl: Add support for encoding sub-messages

Add sub-message encoding to ynl. This makes it possible to create
tc qdiscs and other polymorphic netlink objects.

Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Link: https://lore.kernel.org/r/20240129223458.52046-6-donald.hunter@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>