Calindori online synchronization

I am starting this issue following a discussion with @rinigus and @jbbgameich in the plasma mobile matrix channel.

About a year ago, there was a kind of consensus that a decent solution would be something like this:

1. Create a plugin interface that every calendar data source (e.g. ical file, sink back-end, other) should implement. @nicolasfella has created a review request in kcalendarcore about this some months ago. He has also implemented in !37 the Calindori the plugin mechanism for the local files datasource.

2. Calindori would load the calendar plugins available and offer the calendars of the plugins to the users.

3. Following the kpeoplesink pattern that exposes sink contacts through kpeople to applications like Plasma Mobile Phonebook, a calendar-sink implementation would expose sink calendars through the new calendar plugin interface to applications (like Calindori). I thought that @rpatwal had already worked on that, but I cannot find any related repository.

4. Nextcloud calendar service should be added to kaccounts-providers services and the NextcloudWizard KAccountsUiPlugin should be updated accordingly.

Is this still the plan or there is any alternative? Do I miss any work already done? Please for your feedback.