[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/docstores/base.py

@@ -0,0 +1,54 @@
+from abc import ABC, abstractmethod
+from pathlib import Path
+from typing import List, Optional, Union
+
+from ..documents.base import Document
+
+
+class BaseDocumentStore(ABC):
+    """A document store is in charged of storing and managing documents"""
+
+    @abstractmethod
+    def __init__(self, *args, **kwargs):
+        ...
+
+    @abstractmethod
+    def add(
+        self,
+        docs: Union[Document, List[Document]],
+        ids: Optional[Union[List[str], str]] = None,
+        exist_ok: bool = False,
+    ):
+        """Add document into document store
+
+        Args:
+            docs: Document or list of documents
+            ids: List of ids of the documents. Optional, if not set will use doc.doc_id
+            exist_ok: If True, will not raise error if document already exist
+        """
+        ...
+
+    @abstractmethod
+    def get(self, ids: Union[List[str], str]) -> List[Document]:
+        """Get document by id"""
+        ...
+
+    @abstractmethod
+    def get_all(self) -> dict:
+        """Get all documents"""
+        ...
+
+    @abstractmethod
+    def delete(self, ids: Union[List[str], str]):
+        """Delete document by id"""
+        ...
+
+    @abstractmethod
+    def save(self, path: Union[str, Path]):
+        """Save document to path"""
+        ...
+
+    @abstractmethod
+    def load(self, path: Union[str, Path]):
+        """Load document store from path"""
+        ...