tools: ynl: generate code for ovs families

Add ovs_flow, ovs_vport and ovs_datapath to the families supported
in C. ovs-flow has some circular nesting which is fun to deal with,
but the necessary support has been added already in the previous
release cycle.

Add a sample that proves that dealing with fixed headers does
actually work correctly.

Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Link: https://lore.kernel.org/r/20240202004926.447803-3-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>