fun isConferenceStillPending(invitationId: String): Single<Boolean>
Checks if a conference associated with a specific invitationId is still pending.
invitationId - Identifier of the invitation associated with the conference.
Return
A single that will emit whether the conference is still pending (true) or not (false) when
the request is processed.
In case of error, it will emit a ConferenceError.