[Request]: Rework `Interface` to support non-default routes.

[jglogan]

Feature or enhancement request details

#156 made the gateway IP in Interface optional so the client could set up multiple interfaces with a single default route. We should enhance the struct so that users can set up that configure custom routing on different interfaces.

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[RahulThennarasu]

Hi @jglogan,

I'd like to work on adding custom routing support to Interface.

Question: The existing IpRouteAddLink proto supports direct routes (ip route add <dest> dev <interface>), but not routes with gateways (ip route add <dest> via <gateway>).

Should I:

1. Add optional gateway field to existing IpRouteAddLinkRequest, or
2. Add new IpRouteAddVia RPC method?

Also - IPv4 only initially, or include IPv6?

Thanks!