made GraphQLSpringServerWebExchangeContext public rather than internal so that it can be referenced by dependent project - fixes graphql-java-kickstart#426

Author: bsara

graphql-kickstart-spring-webflux/src/main/java/graphql/kickstart/spring/webflux/GraphQLSpringWebSocketSessionContext.java

@@ -3,7 +3,7 @@
 import graphql.kickstart.execution.context.GraphQLContext;
 import org.springframework.web.reactive.socket.WebSocketSession;
 
-interface GraphQLSpringWebSocketSessionContext extends GraphQLContext {
+public interface GraphQLSpringWebSocketSessionContext extends GraphQLContext {
 
   WebSocketSession getWebSocketSession();