This PR brings support of a shared port between stun (ICE agent) and
the kernel WireGuard
It implements a single port mode for execution with kernel WireGuard
interface using a raw socket listener.
BPF filters ensure that only STUN packets hit the NetBird userspace app
Removed a lot of the proxy logic and direct mode exchange.
Now we are doing an extra hole punch to the remote WireGuard
port for best-effort cases and support to old client's direct mode.
This PR adds supports for the WireGuard userspace implementation
using Bind interface from wireguard-go.
The newly introduced ICEBind struct implements Bind with UDPMux-based
structs from pion/ice to handle hole punching using ICE.
The core implementation was taken from StdBind of wireguard-go.
The result is a single WireGuard port that is used for host and server reflexive candidates.
Relay candidates are still handled separately and will be integrated in the following PRs.
ICEBind checks the incoming packets for being STUN or WireGuard ones
and routes them to UDPMux (to handle hole punching) or to WireGuard respectively.
