Finding caldav parameters through config files, environmental variables or others

[tobixen]

In my plann- and calendar-cli projects, I have defined a configuration file .config/calendar.conf. I came to think, why not move this logic over to the caldav library? My idea was always that it should be possible to use .config/calendar.conf across multiple applications. Moving the logic to the caldav library seems like a good idea. It should also be possible to ship parameters through environmental variables.

• define the API
• Reading parameters through the environment should be trivial to fix
• Move logic for reading config file and finding calendars from the plann application.
• Official name of the config file should be .config/caldav/calendar.conf, with backward-compatibility fallback to .config/calendar.conf. Suffixes .yaml, .json and posisbly .jsonc should also be honored (what about toml, pkl, etc etc?).
• Move/merge logic from tests.conf.client - I will move this to a separate issue Refactor the test configuration again #509

[tobixen]

This seems to be required for easier examples and doctests, so I will prioritize this one.