SpringBoot Update initialization of WebSocket handler in GraphQlWebMvcAutoConfiguration

GraphQlWebSocketHandler now exposes a method to create the WebSocketHttpRequestHandler instance to use, that's pre-configured with a HandshakeInterceptor for context propagation. This is a result of changes for https://github.com/spring-projects/spring-graphql/issues/342.

This pull request updates how the WebSocketHttpRequestHandler is created in the auto configuration, making use of this new method, in order to complete the fix for https://github.com/spring-projects/spring-graphql/issues/342.