Implement routing protocol for partial meshes

[stv0g]

Currently, cunicu expects to build a full mesh between all peers to establish full reachability.
In cases where peer connections can not be directly established, we fall back to TURN relays which transparently proxy the Wireguard UDP packets over a third party.

Alternatively, we could also limit ourself to a partial mesh and use existing peer connections to route trafic to other peers.
This would require the implementation of a routing protocol between the peers.

This approach is also used by Tinc-VPN.

Advantages

• We could easily implement a signaling backend over the partial mesh (see Add new signaling backend by routing messages via already connected peers through established tunnels #9)
• We could limit the number of active peer connections as establishing dedicated peer connections could not scale for a large number of peers

Disadvantages

• Routed trafic would be interceptable by the peers which as a routing hop
  • In contrast: TURN relays would only see encrypted trafic

Open Questions

• Which routing protocol to use?
  • Should we use an established protocol like IS-IS or BGP and embed an routing daemon into cunicu?
  • Existing implementations in Go:
    • bio-rd
    • gobgp
  • Or should we implement a custom protocol based on a Link-state or Path-vector routing scheme?
    • Implementing our own would allow us to customize it for the Wireguard / WICE environemnt
      • Use public keys instead of IP addresses to build a routing table
      • Use gRPC as a transport like wice is using for most other signaling and RPC tasks.

[stv0g]

This is also related to #9

[stv0g]

I've decided to go with a new implementation of the Babel routing protocol: https://github.com/stv0g/go-babel