Use enum approach in builder options to make default, disabled, custom arguments clearer

[elfenpiff]

Brief feature description

Currently, service QoS settings can be handled by:

1. explicitly stating them and make them a requirement when a service is opened
2. not stating them and implicitly use the default values used by the service.

The idea is to introduce some kind of enum to make this more visible in the API.

let service = node.service_builder().some_setting(SettingEnum::Default).another_setting(SettingEnum::Require(123).create();

This issue originated from the discussion: #552 (comment)