Mark UDP socket as ready for write in send msg. (project-chip#7927)

When SendMsg is called, it may create a new socket which is
not part of the files monitored by the system. The FD won't
be added to this list until select (or other monitoring mechanism)
unblock.
This patch ensure that a socket created with SendMsg is added
immediately to the list of FD being monitored.
It mirrors what is done in the Listen function.

src/inet/UDPEndPoint.cpp

@@ -649,6 +649,10 @@ INET_ERROR UDPEndPoint::SendMsg(const IPPacketInfo * pktInfo, System::PacketBuff
     SuccessOrExit(res);
 
     res = IPEndPointBasis::SendMsg(pktInfo, std::move(msg), sendFlags);
+
+    // Wait for ability to write on this endpoint.
+    mSocket.SetCallback(HandlePendingIO, reinterpret_cast<intptr_t>(this));
+    mSocket.OnRequestCallbackOnPendingWrite();
 #endif // CHIP_SYSTEM_CONFIG_USE_SOCKETS
 
 #if CHIP_SYSTEM_CONFIG_USE_NETWORK_FRAMEWORK