tools: ynl: generate code for rt-route and add a sample

YNL C can now generate code for simple classic netlink families.
Include rt-route in the Makefile for generation and add a sample.

    $ ./tools/net/ynl/samples/rt-route
    oif: wlp0s20f3        gateway: 192.168.1.1
    oif: wlp0s20f3        dst: 192.168.1.0/24
    oif: vpn0             dst: fe80::/64
    oif: wlp0s20f3        dst: fe80::/64
    oif: wlp0s20f3        gateway: fe80::200:5eff:fe00:201

Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://patch.msgid.link/20250410014658.782120-14-kuba@kernel.org


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