Even more flexible sources

[silvanmelchior]

For now, ConfZSources are isolated dataclasses with a common (empty) base class. However, this could be improved:

• ConfZSources could be ConfZ classes again, so can use config_cources keyword argument. This would allow e.g. in ConfZFileSource to not have a file_from_cl and file_from_env but rather file only and then use the sources to populate this e.g. from the environment
• The ConfZSource bae class could have common attributes like allow, deby, allow_all, prefix, map, ... with the corresponding logic in the Loader base class. So, this functionality would be supported by all loaders.