Set custom handshakeHandler for XML sockjs config

Prior to this commit, configuring a custom handshakeHandler when setting
up a stomp-endpoint with SockJS would not be taken into account:

  <websocket:stomp-endpoint path="/foo">
    <websocket:handshake-handler ref="customHandler"/>
    <websocket:sockjs/>
  </websocket:stomp-endpoint>

This commit fixes this by creating and registering a
WebsocketTransportHandler (with this handshakeHandler) as a
transportHandler override for the SockJSService.

Issue: SPR-11568