IATP: Implement Credential Store

[paullatzelsperger]

WHAT

Implement a CredentialStore that serves as data retention service for VerifiableCredentialResource entities. The structure of the VerifiableCredentialResource is defined in the architecture documentation

The following operations are needed:

• create: insert a new VC. Will be relevant for VC issuance
• query(QuerySpec): get VCs for read access
• update: overwrite a VC
• delete: remove a VC from storage

The implementation should contain at least an in-mem backend, Postgres will come later.

WHY

The identity hub needs to be able to store VerifiableCredential entities.