Make API layer implementation support async and streaming output mode

[imbajin]

refer: https://github.com/apache/incubator-hugegraph-ai/tree/main/hugegraph-llm/src/hugegraph_llm/api

At present, we only support synchronous return, without asynchronous implementation or the option to add streaming output

[chiruu12]

Can i work on this issue?

[chiruu12]

Do i also have to do the TODO part in the rag_api.py file?

[imbajin]

rag_api.py

@chiruu12 Not necessary, it's not important at the moment(fewer calls), we can handle it later