Better Introspection (#501)

- Add all missing database fields to `WorkflowStatus`
- Add `forked_from` and `forked_to` fields to `WorkflowStatus`, tracing
workflow fork lineage and closing
https://github.com/dbos-inc/dbos-transact-py/issues/498
- Use `pool_pre_ping` in the client, closing
https://github.com/dbos-inc/dbos-transact-py/issues/500
- Record step start and end times, enabling visualization of step
duration