FR: allow for additional CalDAV providers

[zenodotus280]

Is your feature request related to a problem?

I have access to more than one CalDAV server but I must choose just one in Cfait. One workaround would be to use an alternative config as described in #63 but plurality of CalDAV backends would be better.

Describe the solution you'd like

Three potential options come to mind (in order of preference):

1. Utilize vdirsyncer on desktop and DAVx5 on Android to reduce the amount of manual configuration.
2. Implement new logic to handle multiple backends.
3. Alternative config files as a desktop-only workaround.

Additional context (optional)

Option 1 also benefits people who only ever use one backend but already use CalDAV in other apps (e.g. khard for CLI contacts or regular contacts on Android)

[trougnouf]

I worked on this for a while because I regularly needed to test different servers ( https://github.com/trougnouf/cfait/tree/feature/multiple_servers ) but it's a pretty complex refactor (which makes the codebase a lot more complex) and I never got a satisfying interface so I settled on different root paths.

1. I want to keep the same core on desktop and Android (and I much prefer Cfait handling its own sync; it's a direct one-step process and it has much more control over conflict / synchronization issues)
2. Per above. I'm not against it but I'm unlikely to work on this again anytime soon because running two instances works relatively well.
3. That's already implemented (and FR: Have config specify location of the cfait data #63 is not what you want here, it's asking about a different data path for a given config), you can run cfait or cfait-gui instances with the --root <path> parameter (though the Android client is lacking here at the moment).

[trougnouf]

The least complex way to do this on Android would be a switch which requires restarting the app. Let me know if you'd like that.
(Alternatively I could just ship two APKs for two instances, that's a hack but it comes with no added complexity in the codebase and allows two instances to run simultaneously like on desktop.)

[zenodotus280]

No, all good for now. It's only on desktop that I'd want access to multiple servers so --root will work fine.

[pcause]

perhaps besides having the location in the config, a datafile command in the tui/gui can specify a new datafile to use for display. assume the same config but just different source. kind of like basically re-initialize with current config but override the datafile location in the config to the one specified in the command.

[pcause]

might also allow multiple sources with names in the config. config.datalocation.default, config.datalocation.mycaldav, config.datalocationgooglecal, etc. on start you use config.default but the new datalocation takes a name and uses location with that name,