[Bug]: MilvusClient.search doesn't support _async

[drawnwren]

Is there an existing issue for this?

• I have searched the existing issues

Describe the bug

When I do MilvusClient().search(...., _async=True, ...) I get an exception:

Expected Behavior

I expect MIlvusClient().search(_async=True) to return a SearchFuture

Steps/Code To Reproduce behavior

I've added a PR to address this #2369

Environment details

- Hardware/Softward conditions (OS, CPU, GPU, Memory):
- Method of installation (Docker, or from source):
- Milvus version (v0.3.1, or v0.4.0):
- Milvus configuration (Settings you made in `server_config.yaml`):

Anything else?

No response