Wrong return type on create_timer()

🐛 **Describe the bug**
Type hint for return value of `create_timer()` is `TaskBase`, but that does not contain the `cancel()` function. ~Returning `TimerTask` seems more appropiate.~

_Edit: See comment below on why `TimerTask` might not be a viable type anyway_

Current source:

https://github.com/Azure/azure-functions-durable-python/blob/4ab004b6bc994b0395ee965186f8402b86b7a887/azure/durable_functions/models/DurableOrchestrationContext.py#L601

Example code:

```python
expiration = context.current_utc_datetime + timedelta(seconds=20)
timeout_task = context.create_timer(expiration)
timeout_task.cancel() # <-- Pylance shows error: "Cannot access attribute "cancel" for class "TaskBase""
```

🤔 **Expected behavior**
You should be able to call `cancel()`without linter/type checker errors.

☕ **Steps to reproduce**
See above

Start at azure/durable_functions/models/DurableOrchestrationContext.py around line 601 and inspect the return annotation for create_timer(). Reproduce the example with Pylance, then verify that calling cancel() no longer produces a type-checking error and that the annotation accurately represents the returned timer task.

Scritto dal modello di indicizzazione a partire dal testo della issue.