Schedule task to derive code mappings when a new group is found

We currently schedule the task to derive code mappings too often. I want to reduce this unnecessary load.

We currently schedule tasks in [post_process](https://github.com/getsentry/sentry/blob/369f09eaef4b7658ad4ad7293ce98fa009fd4b98/src/sentry/tasks/post_process.py#L1020) (700M+ events in the last day), however, I want to move it to when we need to [find a group for the event](https://github.com/getsentry/sentry/blob/369f09eaef4b7658ad4ad7293ce98fa009fd4b98/src/sentry/event_manager.py#L1289-L1298) (in the last 24 hours about 0.5% of events create a group).

[*] Data source "Seer Grouping" Datadog dashboard