feature/graph db/interfaces

This merge request contains the Graph DB interfaces for storing and querying self-description claims. Specifically, it includes:

• GraphStore interface for CRUD operations
• QueryGraph interface for query operations

Moreover, we updated the following POJO classes:

• SdClaim: POJO Class for holding a Claim. A Claim is a triple represented by a subject, predicate, and object.
• OpenCypherQuery: POJO Class for holding a Cypher Query.