add DBGPTRetriever for langchain

[Aries-ckt]

I want add DBGPTRetriever for langchain this is my pr:#7537.
but says if EmbeddingEngine it make more sense to implement as a VectorStore instead of a Retriever?
db-gpt is not a vector store, it's a vector connector service like ChaindeskRetriever, it can connect milvus, weaviate, elasticsearch db, so db-gpt just provide document retriever service. Maybe it's make more sense to as a Retriever?
@baskaryan @rlancemartin, @eyurtsev, @hwchase17