Add ZepMemory; improve ZepChatMessageHistory handling of metadata; Fix bugs (#7444)

Hey @hwchase17 - 

This PR adds a `ZepMemory` class, improves handling of Zep's message
metadata, and makes it easier for folks building custom chains to
persist metadata alongside their chat history.

We've had plenty confused users unfamiliar with ChatMessageHistory
classes and how to wrap the `ZepChatMessageHistory` in a
`ConversationBufferMemory`. So we've created the `ZepMemory` class as a
light wrapper for `ZepChatMessageHistory`.

Details:
- add ZepMemory, modify notebook to demo use of ZepMemory
- Modify summary to be SystemMessage
- add metadata argument to add_message; add Zep metadata to
Message.additional_kwargs
- support passing in metadata

tests/unit_tests/memory/chat_message_histories/test_zep.py

@@ -4,7 +4,7 @@ import pytest
 from pytest_mock import MockerFixture
 
 from langchain.memory.chat_message_histories import ZepChatMessageHistory
-from langchain.schema.messages import AIMessage, HumanMessage
+from langchain.schema.messages import AIMessage, HumanMessage, SystemMessage
 
 if TYPE_CHECKING:
     from zep_python import ZepClient
@@ -39,7 +39,7 @@ def test_messages(mocker: MockerFixture, zep_chat: ZepChatMessageHistory) -> Non
     result = zep_chat.messages
 
     assert len(result) == 3
-    assert isinstance(result[0], HumanMessage)  # summary
+    assert isinstance(result[0], SystemMessage)  # summary
     assert isinstance(result[1], AIMessage)
     assert isinstance(result[2], HumanMessage)