Provide an API to interact with CalDav server

[HoussemNasri]

What?

To implement #1329, we need to interact with the CalDav server in order to get/set event attendance status. This ticket is preliminary work for the mentioned ticket.

How?

Introduce DavClient which encapsulates CardDav and CalDav operations used in other parts of the codebase.

public interface DavClient {
    Mono<Boolean> existsCollectedContact(String username, String userId, String collectedId);

    Mono<Void> createCollectedContact(String username, String userId, CardDavCreationObjectRequest creationObjectRequest);

    Mono<PartStat> getUserParticipationStatus(String userId, String eventUuid);

    Mono<Void> setUserParticipationStatus(String userId, String eventUuid, PartStat partStat);
}

Definition of done

Integration tests for getUserParticipationStatus and setUserParticipationStatus

[chibenwa]

eventUuid => eventUid As I recall I do not think the spec mandates a uuid, just a globally unique identifier.