Share kernel Wireguard port with raw socket (#826)

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.

client/internal/proxy/direct.go

@@ -1,57 +0,0 @@
-package proxy
-
-import (
-	log "github.com/sirupsen/logrus"
-	"net"
-)
-
-// DirectNoProxy is used when there is no need for a proxy between ICE and WireGuard.
-// This is possible in either of these cases:
-// - peers are in the same local network
-// - one of the peers has a public static IP (host)
-// DirectNoProxy will just update remote peer with a remote host and fixed WireGuard port (r.g. 51820).
-// In order DirectNoProxy to work, WireGuard port has to be fixed for the time being.
-type DirectNoProxy struct {
-	config Config
-	// RemoteWgListenPort is a WireGuard port of a remote peer.
-	// It is used instead of the hardcoded 51820 port.
-	RemoteWgListenPort int
-}
-
-// NewDirectNoProxy creates a new DirectNoProxy with a provided config and remote peer's WireGuard listen port
-func NewDirectNoProxy(config Config, remoteWgPort int) *DirectNoProxy {
-	return &DirectNoProxy{config: config, RemoteWgListenPort: remoteWgPort}
-}
-
-// Close removes peer from the WireGuard interface
-func (p *DirectNoProxy) Close() error {
-	err := p.config.WgInterface.RemovePeer(p.config.RemoteKey)
-	if err != nil {
-		return err
-	}
-	return nil
-}
-
-// Start just updates WireGuard peer with the remote IP and default WireGuard port
-func (p *DirectNoProxy) Start(remoteConn net.Conn) error {
-
-	log.Debugf("using DirectNoProxy while connecting to peer %s", p.config.RemoteKey)
-	addr, err := net.ResolveUDPAddr("udp", remoteConn.RemoteAddr().String())
-	if err != nil {
-		return err
-	}
-	addr.Port = p.RemoteWgListenPort
-	err = p.config.WgInterface.UpdatePeer(p.config.RemoteKey, p.config.AllowedIps, DefaultWgKeepAlive,
-		addr, p.config.PreSharedKey)
-
-	if err != nil {
-		return err
-	}
-
-	return nil
-}
-
-// Type returns the type of this proxy
-func (p *DirectNoProxy) Type() Type {
-	return TypeDirectNoProxy
-}