Added client_settings support for chromadb vecstore (#1528)
# Problem
The ChromaDB vecstore only supported local connection. There was no way
to use a chromadb server.
# Fix
Added `client_settings` as Chroma attribute.
# Usage
```
from chromadb.config import Settings
from langchain.vectorstores import Chroma
chroma_settings = Settings(chroma_api_impl="rest",
chroma_server_host="localhost",
chroma_server_http_port="80")
docsearch = Chroma.from_documents(chunks, embeddings, metadatas=metadatas, client_settings=chroma_settings, collection_name=COLLECTION_NAME)
```
