Pluck replay_id out of dynamic sampling context and add it to context if it exists

[JoshFerge]

As part of Connecting Backend Errors With Replays, https://github.com/getsentry/replay-backend/issues/251, we decided that we will add replay_id to the dynamic sampling context (getsentry/sentry-javascript#7555).

In Relay, we should pluck the replay_id out of the dynamic sampling context and add it to the context of the event. This will then be written as a column in the various clickhouse tables. I will plan on making a PR in one of our next sprints.

Original RFC getsentry/rfcs#60