Replay events should have a unique `event_id`

[Lms24]

Problem Statement

Currently, we're assigning the replayId as event_id in each Replay event we send to Sentry. All events/segments of one Replay session, have therefore the same event_id. This works because we do not process Replay events in the same way as other events, which is why we don't strictly need unique event ids for each event. However, we should nevertheless change this as it could end up becoming problematic if we ever change something around Replay event processing. Furthermore, it's very confusing and an easy fix.

Solution Brainstorm

Assign a unique uuid4 event_id, just like we do with all other events.