DBOS.get_events (#477)

Adds new methods that retrieve all the published events for a given
workflow.

```python
def get_all_events(workflow_id: str) -> Dict[str, Any]:
async def get_all_events_async(workflow_id: str) -> Dict[str, Any]:
```

Addresses https://github.com/dbos-inc/dbos-transact-py/issues/476