Implement CRUD APIs for subscription entity

[kmrdhruv]

• Implement CRUD APIs, which interact with metastore in terms of persistence and retrieval.
• Functional implementation will be added independently of this task.
• They can be started as simple entities w/o initial set of policies. Entities can be enhanced with functional implementation to include various subscription related policies.
• Add tests for code changes, including E2E.

Requirements are captured in https://docs.google.com/document/d/1SmdBOodu4Mr4L9bdrpA0SFgzuueaIW_GU7KcBWyJTmw/edit

[kmrdhruv]

moving it to current sprint for tracking/closure.