Add method for adding/removing servlet listeners.

Author: apottere

src/main/java/graphql/servlet/GraphQLServlet.java

@@ -93,6 +93,14 @@ public void removeOperationListener(GraphQLOperationListener operationListener)
         operationListeners.remove(operationListener);
     }
 
+    public void addServletListener(GraphQLServletListener servletListener) {
+        servletListeners.add(servletListener);
+    }
+
+    public void removeServletListener(GraphQLServletListener servletListener) {
+        servletListeners.remove(servletListener);
+    }
+
     @Override
     public String[] getQueries() {
         return getSchema().getQueryType().getFieldDefinitions().stream().map(GraphQLFieldDefinition::getName).toArray(String[]::new);