This implementation should simplify future changes to configuration entries.
Error handling was shifted towards special loading contexts.

The core concepts are:

• There is no need for a long list of backing fields anymore. New config entry "getters" just need to be marked with @config_entry
• Loading entries from file uses the ConfigDSL, which provides the methods get_or_none, get_with_default and get_or_raise. These methods can accept default args and format strings for success or failure.
• The internal config entry naming scheme was slightly altered: Since "getters" are now enriched with further attributes, the "get" prefix was dropped. Entries representing bool flags now begin with "enable" instead of "turn_on"

Checklist:

• I made sure that the CI passed before I ask for a review.
• I added a summary of the changes (compared to the last release) in the CHANGELOG.md.
• If necessary, I made changes to the documentation and/or added new content.
• I will remember to squash-and-merge, providing a useful summary of the changes of this PR.