Customized milvus schema is not supported in LangChain mode

[yuanjie-ai]

https://github.com/zilliztech/akcio/blob/a48121d8fb93765ed7cc2de28de29ad5504ae117/src_langchain/store/vector_store/milvus.py

[jaelgu]

Supported in Towhee mode by pr #46.
For LangChain mode, it requires code change in LangChain source code. I will update the method once LangChain supports the feature.