Merge branch 'ynl-add-support-for-user-headers-and-struct-attrs'

Donald Hunter says:

====================
ynl: add support for user headers and struct attrs

Add support for user headers and struct attrs to YNL. This patchset adds
features to ynl and add a partial spec for openvswitch that demonstrates
use of the features.

Patch 1-4 add features to ynl
Patch 5 adds partial openvswitch specs that demonstrate the new features
Patch 6-7 add documentation for legacy structs and for sub-type
====================

Link: https://lore.kernel.org/r/20230327083138.96044-1-donald.hunter@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>