Feature Request: Finish Implementation of the Graph Adapter

[vonjackets]

It is a goal of Polar to be somewhat agnostic to whichever graph database a user chooses to store data in. Work to implement the graph adapter traits and data structures started during the project's initial development phase but were not completed. So as of now the agents rely on neo4j via the neo4rs crate. While useful, it is unsatisfactory. Finishing the graph adapter implementation and wrapping neo4rs in a particular wrapper would be a great start to making the project more modular.