doc/netlink: Add spec for rt route messages

Add schema for rt route with support for getroute, newroute and
delroute.

Routes can be dumped with filter attributes like this:

./tools/net/ynl/cli.py \
    --spec Documentation/netlink/specs/rt_route.yaml \
    --dump getroute --json '{"rtm-family": 2, "rtm-table": 254}'

Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://lore.kernel.org/r/20230825122756.7603-13-donald.hunter@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>