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

从 azure/durable_functions/models/DurableOrchestrationContext.py 约第 601 行开始，检查 create_timer() 的返回注解。使用 Pylance 重现该示例，然后验证调用 cancel() 不再产生类型检查错误，并确认该注解准确表示返回的计时器任务。

由索引模型根据 Issue 内容生成。