- Add SetResult enum with Inserted and AlreadyExists variants
- Update KeyValueStorage::set() to return Result<SetResult> instead of Result<()>
- Change behavior: return AlreadyExists instead of error when key exists and overwrite=false
- Update all storage implementations (LocalFs, LocalJson, Memory, Postgres)
- Update PolicyEngine::set_policy() to handle new return type

This change allows callers to distinguish between successful insertion
and existing key scenarios without treating 'already exists' as an error.

Signed-off-by: Xynnn007 <[email protected]>

…pport

This commit removes the Engine trait abstraction in favor of concrete,
scenario-specific implementations. The rationale is:

1. Different policy engines may have different entry parameters and APIs,
   making a unified trait interface impractical and overly complex.

2. Policy engines are tightly coupled to their specific use cases. For
   example, the AS (Attestation Service) for EAR token issuance uses
   Rego policies, and there's no foreseeable need to support other
   policy languages in the near future.

3. By creating concrete engine instances at their usage sites and
   encapsulating the implementation details, we achieve better type
   safety and clearer code organization without unnecessary abstraction.

To achieve these goals, this commit

- Refactor PolicyEngine from trait-based to generic design (PolicyEngine<T>)
- Remove Engine trait and PolicyType enum
- Enhance Regorus engine with extension support
- Add ExtensionWrapper struct for Regorus extensions
- Make data parameter optional in Regorus::evaluate
- Add AddRegorusExtensionFailed error variant
- Update error comments from OPA to Regorus terminology
- Move engine-specific methods to PolicyEngine<Regorus> implementation

Signed-off-by: Xynnn007 <[email protected]>