Create a shared structure for Feast secret manifest and FeastStoreConfiguration dataclass #13

Context

In #9 the library defines a FeastStoreConfiguration dataclass, this class has an identical structure to the Feast secret template. In the PR review #9 (comment) we noticed that this structure is duplicated and it would be a better design to share this structure among the charm and the library. The structure was not placed in the library because the library's scope is limited to handling the relation. This issue is to discuss how we can centralize the structure, and what should be the entity to encapsulate it, some suggestions were:

• in the existing charm lib
• in the charm code
• in a template owned by the charm

What needs to get done

1. Discuss with the team where this kind of shared entity between charm and charm code should exist
2. Implement the suggested approach

Definition of Done

Feast store configuration structure is no longer duplicated across the secret template and the library code