Modify Sentry SDK to have a "replay" context, add replay_id to this context for transactions and errors

Fulfilling [this RFC](https://github.com/getsentry/rfcs/blob/main/text/0048-move-replayid-out-of-tags.md), we want to create a new context `replay` on the Sentry event schema. this context then contains the `replay_id` for events with replays associated. (this does not impact replay_events themselves for now).