create or get instance of orchestrator

I was creating a orchestration that was waiting for three events (from EventGrid) and then calls another two activities and got it working, but noticed that my code is needlessly complex, this is what happens in my trigger (with EventHubTrigger), after creating a blob object with a order_id attribute, I need this code:
```python
try:
status = await client.get_status(blob.order_id)
instance_id = status.instance_id
except Exception:
instance_id = await client.start_new("Orchestrator", blob.order_id, None)
if instance_id:
await client.raise_event(instance_id, blob.file_type, True)
else:
logging.error("Could not start orchestrator for instance: %s", blob.order_id)
```

I would expect either the `client.start_new` call to either create a new one or get an existing one, or for there to be a something like a `get_or_start_new` function on client.

The reason I needed to do it like this is that I have a single input (EventGrid triggered by storage) and need to both start a orchestration and raise an event, I do have an instance_id I can use to align the different events to the same orchestrator instance but the fact that I can't just have this function call `start_new` three times is very unintuitive and hard to debug.

首先阅读 issue 中提到的客户端方法：get_status、start_new 和 raise_event。定义当多个事件针对同一个 order_id 时，创建或获取操作应如何表现，然后验证生成的实例无需调用方执行 status-and-start 序列即可接收所请求的事件。

由索引模型根据 Issue 内容生成。