private ConfigDefinition getStructDef(String name) {
        return (configDefinition == null ? null : configDefinition.getStructDefs().get(name));
    }
Get a new payload builder for a config struct, which can be used to add inner values to that struct.

@param name name of the struct to create
@return a payload builder corresponding to the name
config/src/main/java/com/yahoo/vespa/config/ConfigPayloadBuilder.java