tools: ynl: generate code for ovs families

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 <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>