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.

sharedsock/sock_nolinux.go

@@ -0,0 +1,14 @@
+//go:build !linux
+
+package sharedsock
+
+import (
+	"fmt"
+	"net"
+	"runtime"
+)
+
+// Listen is not supported on other platforms
+func Listen(port int, filter BPFFilter) (net.PacketConn, error) {
+	return nil, fmt.Errorf(fmt.Sprintf("Not supported OS %s. SharedSocket is only supported on Linux", runtime.GOOS))
+}