netlink: specs: add ynl spec for ovs_flow

Add a ynl specification for ovs_flow. This spec is sufficient to dump ovs
flows. Some attrs are left as binary blobs because ynl doesn't support C
arrays in struct definitions yet.

Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>