To support a simple API in relay (as part of #49203) we will need to have some way for the consumer to handle checkins that do not have paired check-in IDs. This is because we do not want users to have to keep track of a check_in ID for their script, instead they should be able to make the assumption that the ok and error will update the most recent in_progress checkin.

To handle this we should support receiving a check_id id as the string value latest.

For consistency we can accept this for all status, but it should never re-open a closed checkin. Thus if the latest checkin was already at a terminal state, there is no latest and a new checkin will be created.

As it was implemented in getsentry/relay#2153 we'll want to support this as a empty UUID to indicate latest.