Implement hasParticipant() on ActivityInterface

Problem/Motivation

In #2705569: Check for an existing activity before creating new one we want to work with activity revisions and we need a method to check if an activity already has a given contact/participant.
Not sure if this is going to be still valid once we update a data model.

Proposed resolution

Implement a method Activity::hasParticipant() method that should loop over all participants and check if a given one already exists in the list.