Implement tag-based policy re-check in handlers

- Add checkPolicyWithEntry helper to S3ApiServer for handlers to re-check
  policy after fetching object entry (for s3:ExistingObjectTag conditions)
- Add HasPolicyForBucket method to policy engine for efficient check
- Integrate policy re-check in GetObjectHandler after entry is fetched
- Integrate policy re-check in HeadObjectHandler after entry is fetched
- Update auth_credentials.go comments to explain two-phase evaluation
- Update documentation with supported operations for tag-based conditions

This implements 'Approach 1' where handlers re-check the policy with
the object entry after fetching it, allowing tag-based conditions to
be properly evaluated.

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