Feature Request: Chat memory classes that also persist Summaries in DB

[ncfx]

Would love to have memory classes that can enable the ConversationSummaryBufferMemory to persist the Summary to a DB in addition to the chat messages. Right now we have memory classes like PostgresChatMessageHistory that only persist the chat histories.

I've been trying to build a nice clean PR with the changes, but I can't get around Pydantic validation issues when extending the langchain classes, and it's just been a real drag with no clue as to why straightforward things don't work (sorry I'm a Pydantic noob), so I had to edit the langchain classes directly to get summary persistence to work and just move forward to make progress. The direct edits were really straightforward. Trying to do things the right way is not straightforward and I don't have time to figure out why. If someone can do the PR, it would be greatly appreciated and I can learn something as well.

Thanks