make BaseEntityStore inherit from BaseModel (#5478)

# Make BaseEntityStore inherit from BaseModel

This enables initializing InMemoryEntityStore by optionally passing in a
value for the store field.

## Who can review?

It's a small change so I think any of the reviewers can review, but
tagging @dev2049 who seems most relevant since the change relates to
Memory.

langchain/memory/entity.py

@@ -3,7 +3,7 @@ from abc import ABC, abstractmethod
 from itertools import islice
 from typing import Any, Dict, Iterable, List, Optional
 
-from pydantic import Field
+from pydantic import BaseModel, Field
 
 from langchain.base_language import BaseLanguageModel
 from langchain.chains.llm import LLMChain
@@ -19,7 +19,7 @@ from langchain.schema import BaseMessage, get_buffer_string
 logger = logging.getLogger(__name__)
 
 
-class BaseEntityStore(ABC):
+class BaseEntityStore(BaseModel, ABC):
     @abstractmethod
     def get(self, key: str, default: Optional[str] = None) -> Optional[str]:
         """Get entity value from store."""