Following #528, we really need a way to store state after each node is run.

We should provide:

• an ABC for persisting state
• a disk implementation
• a sqlite implementation

In future we'll probably want to provide a postgres implementation, but I think that can come later when we're more confident of the schema.

This also relates to other database access and persistence:

• message persistence in graphs Ability to Persist Messages in External Stores #530
• vector search for RAG Vector search and embeddings API #58