Add AsyncListenSocket, as alias for AsyncPacketSocket

A preparation for splitting server sockets out into a separate
interface, see https://webrtc-review.googlesource.com/c/src/+/232607.

Transition plan:

1. Land this cl.

2. Update downstream code to use the new name.

3. Attempt landing
   https://webrtc-review.googlesource.com/c/src/+/232607. May need
   additional steps to not break downstream implementations of
   PacketSocketFactory::CreateServerTcpSocket.

Bug: webrtc:13065
Change-Id: Ife448c705222f4c9f66a096e3dc7eb07e0f9c3af
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/233700
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#35155}

rtc_base/async_packet_socket.h

@@ -135,6 +135,11 @@ class RTC_EXPORT AsyncPacketSocket : public sigslot::has_slots<> {
   RTC_DISALLOW_COPY_AND_ASSIGN(AsyncPacketSocket);
 };
 
+// TODO(bugs.webrtc.org/13065): Intended to be broken out into a separate class,
+// after downstream has adapted the new name. The main feature to move from
+// AsyncPacketSocket to AsyncListenSocket is the SignalNewConnection.
+using AsyncListenSocket = AsyncPacketSocket;
+
 void CopySocketInformationToPacketInfo(size_t packet_size_bytes,
                                        const AsyncPacketSocket& socket_from,
                                        bool is_connectionless,