Provide a CalDAV based implementation for EventAttendanceRepository

[chibenwa]

Context

A standalone version of Event Attendance Management has been implemented in #1326 that does not require a side-by-side OpenPaas deployment.

In addition, a JMAP API has been introduced to query user event attendance status in #1328, a task that was previously accomplished through parsing email flags.

The final piece of the jigsaw is a DAV-based attendance management feature that receives attendance updates from the deployed DAV server.

Why?

Interacting with OpenPaas to access and update user calendars has proven to be ineffective. This is because the project is not under active development and any issues that arise may not be resolved as quickly as we would like.

The solution is to interact directly with the underlying DAV server. This has the added benefit of being able to interact with any DAV server, not necessarily the OpenPaas one.

How?

[HoussemNasri]

Ticket split proposition:

1. Create DavClient instead of the existing CarDavClient, which should handle CalDav operations.
2. Introduce CalDavEventAttendanceRepository (this ticket)
3. Prepare a demo.

[chibenwa]

We need to be recursion aware.

The buttons needs to match the corresponding CALDAV VEVENT identified by UID and RECURRENCE-ID.