[AUR-420] Provide document store base interface and an in-memory version (#21)

Document store handles storing and indexing Documents. It supports the following interfaces:

- add: add 1 or more documents into document store
- get: get a list of documents
- get_all: get all documents in a document store
- delete: delete 1 or more document
- save: persist a document store into disk
- load: load a document store from disk

knowledgehub/embeddings/base.py

@@ -1,7 +1,7 @@
 from abc import abstractmethod
 from typing import List, Type
 
-from langchain.embeddings.base import Embeddings as LCEmbeddings
+from langchain.schema.embeddings import Embeddings as LCEmbeddings
 from theflow import Param
 
 from ..components import BaseComponent