netlink: add schemas for YAML specs

Add schemas for Netlink spec files. As described in the docs
we have 4 "protocols" or compatibility levels, and each one
comes with its own schema, but the more general / legacy
schemas are superset of more modern ones: genetlink is
the smallest followed by genetlink-c and genetlink-legacy.
There is no schema for raw netlink, yet, I haven't found the time..

I don't know enough jsonschema to do inheritance or something
but the repetition is not too bad. I hope.
Acked-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>