feat: Policy Engine Control Plane Client

[Hweinstock]

Problem

The SDK has no Cedar policy engine support. Customers who need to programmatically create policy engines, manage Cedar policies, or use NL→Cedar policy generation have no SDK path. The starter toolkit's PolicyClient demonstrates the full scope of what the SDK should support: policy engine lifecycle management, Cedar policy CRUD, idempotent create helpers, and a high-level generate_policy() that orchestrates the async generation workflow.

Acceptance Criteria

• Customers can create, get, list, update, and delete policy engines
• Customers can create, get, update, list, and delete Cedar policies within a policy engine
• Customers can start NL→Cedar policy generation, poll for completion, and retrieve generated assets
• Customers can create a policy from a generation asset
• Idempotent create is supported (create_or_get_policy_engine, create_or_get_policy)
• Create operations poll until ACTIVE status
• All functionality is verified via integration tests running in CI

Relevant Links

• PolicyClient
• create_or_get_policy_engine()
• create_policy()
• create_or_get_policy()
• generate_policy()
• create_policy_from_generation_asset()
• cleanup_policy_engine()