[udp] add template SocketIn class for easier socket usage (#10392)

This commit adds a new template `Udp::SocketIn<Owner, RxHandlerPtr>`,
which defines a UDP socket for use within a specified `Owner` class.
It includes a `HandleUdpReceive()` member method callback for
processing received messages. This model, similar to `TimerMilliIn`,
eliminates the need for each socket user to define boilerplate code
for providing a static member function that simply casts and calls
the member method. This is now handled by the template `SocketIn`
class. The `Udp::Socket::Open()` method is also modified to no longer
require the callback and context parameters.

tests/unit/test_srp_server.cpp

@@ -1068,7 +1068,7 @@ void TestSrpClientDelayedResponse(void)
         //- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
         // Prepare a socket to act as SRP server.
 
-        Ip6::Udp::Socket  udpSocket(*sInstance);
+        Ip6::Udp::Socket  udpSocket(*sInstance, HandleServerUdpReceive, nullptr);
         Ip6::SockAddr     serverSockAddr;
         uint16_t          firstMsgId;
         Message          *response;
@@ -1076,7 +1076,7 @@ void TestSrpClientDelayedResponse(void)
 
         sServerRxCount = 0;
 
-        SuccessOrQuit(udpSocket.Open(HandleServerUdpReceive, nullptr));
+        SuccessOrQuit(udpSocket.Open());
         SuccessOrQuit(udpSocket.Bind(kServerPort, Ip6::kNetifThread));
 
         //- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -