Add shortcuts for created_time and last_edited_time

```python
client.databases.query(DB_ID, filters.Timestamp.last_edited_time.after("2022-01-01")
```
instead of 
```python
client.databases.query(DB_ID, filters.Timestamp("last_edited_time").after("2022-01-01")
```

Allows faster/easier coding as the shortcuts will support auto-completion.