feature/relayserver,net/{netcheck,udprelay}: implement addr discovery (#16253)

The relay server now fetches IPs from local interfaces and external
perspective IP:port's via netcheck (STUN).

Updates tailscale/corp#27502

Signed-off-by: Jordan Whited <jordan@tailscale.com>

net/udprelay/server_test.go

@@ -156,7 +156,7 @@ func TestServer(t *testing.T) {
 
 	ipv4LoopbackAddr := netip.MustParseAddr("127.0.0.1")
 
-	server, _, err := NewServer(0, []netip.Addr{ipv4LoopbackAddr})
+	server, _, err := NewServer(t.Logf, 0, []netip.Addr{ipv4LoopbackAddr})
 	if err != nil {
 		t.Fatal(err)
 	}