[EventEngine] Set DSCP on listener socket (#35983)

Prepare the listener socket with the configured DSCP value to mark outgoing packets correctly.
This is already done correctly for clients.

During implementation and delivery of the DSCP feature both experiments `event_engine_listener` and `event_engine_client` were default `OFF`. Unfortunately the `event_engine_listener` experiment was not enabled correctly during verification, which resulted in that the working `iomgr` codepath was used instead.
This PR adds the missing action which I now have verified correctly.

Fixes #35954

Closes #35983

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/35983 from Nordix:fix-dscp 72969db88e
PiperOrigin-RevId: 609770886

src/core/lib/event_engine/posix_engine/posix_engine_listener_utils.cc

@@ -169,6 +169,7 @@ absl::Status PrepareSocket(const PosixTcpOptions& options,
       !ResolvedAddressIsVSock(socket.addr)) {
     GRPC_RETURN_IF_ERROR(socket.sock.SetSocketLowLatency(1));
     GRPC_RETURN_IF_ERROR(socket.sock.SetSocketReuseAddr(1));
+    GRPC_RETURN_IF_ERROR(socket.sock.SetSocketDscp(options.dscp));
     socket.sock.TrySetSocketTcpUserTimeout(options, false);
   }
   GRPC_RETURN_IF_ERROR(socket.sock.SetSocketNoSigpipeIfPossible());