Add function on RouteMessageBuilder for setting route MTU

[MarkusPettersson98]

Title. Found myself doing the following thing in my code:

let message_builder = RouteMessageBuilder::<Ipv4Addr>::new();
// Assemble the RouteMessage
let mut msg: RouteMessage = message_builder.build();
// HACK: Set route MTU on the assembled RouteMessage
// TODO: Submit PR upstream for doing this on the builder instead.
let mtu = RouteMetric::Mtu(1280);
msg.attributes.push(RouteAttribute::Metrics(vec![mtu]));

With this patch, I would not need to create a mutable RouteMessage :)

let msg = RouteMessageBuilder::<Ipv4Addr>::new()
    .mtu(1280)
    .build();

[cathay4t]

This override future changes to other RouteMetric (e.g. cwnd).

Please create similar stuff like RouteNextHopBuilder