[client] Eliminate UDP proxy in user-space mode (#2712)

In the case of user space WireGuard mode, use in-memory proxy between the TURN/Relay connection and the WireGuard Bind. We keep the UDP proxy and eBPF proxy for kernel mode.

The key change is the new wgproxy/bind and the iface/bind/ice_bind changes. Everything else is just to fulfill the dependencies.

client/iface/wgproxy/proxy.go

@@ -0,0 +1,15 @@
+package wgproxy
+
+import (
+	"context"
+	"net"
+)
+
+// Proxy is a transfer layer between the relayed connection and the WireGuard
+type Proxy interface {
+	AddTurnConn(ctx context.Context, endpoint *net.UDPAddr, remoteConn net.Conn) error
+	EndpointAddr() *net.UDPAddr // EndpointAddr returns the address of the WireGuard peer endpoint
+	Work()                      // Work start or resume the proxy
+	Pause()                     // Pause to forward the packages from remote connection to WireGuard. The opposite way still works.
+	CloseConn() error
+}