Segment context not propagated when `asyncio.run_in_executor` is used

(Minimised) Example code:

```python
import asyncio

from aws_xray_sdk.core import xray_recorder
from aws_xray_sdk.core import patch_all
from aws_xray_sdk.core.async_context import AsyncContext

import boto3

xray_recorder.configure(service="Test app", sampling=False, context=AsyncContext())
patch_all()

def run_task():
s3_client = boto3.client("s3")
s3_client.list_buckets()

async def main():
loop = asyncio.get_running_loop()
xray_recorder.begin_segment("test")
await loop.run_in_executor(None, run_task)
xray_recorder.end_segment()

asyncio.run(main())
```

Output:
```
cannot find the current segment/subsegment, please make sure you have a segment open
No segment found, cannot begin subsegment s3.
cannot find the current segment/subsegment, please make sure you have a segment open
cannot find the current segment/subsegment, please make sure you have a segment open
No segment to end
cannot find the current segment/subsegment, please make sure you have a segment open
```

I understand from the README that when using a thread pool (which is the default executor), the trace entity needs to be manually propagated. Could it be propagated automatically?

I'm asking because in the use case I have, a 3rd party library I'm using is making the call so I couldn't patch the call sites easily.

README のスレッドプールでのトレース伝播に関する説明と、loop.run_in_executor とともに使用される AsyncContext のエントリーポイントから始めます。最小化された例を再現し、次に run_task が現在のセグメントを取得する方法を追跡し、伝播が自動の場合に、セグメント不足またはサブセグメント不足のメッセージなしで例が完了することを確認します。

索引モデルが issue の本文から書いたものです。