s3: add s3:ExistingObjectTag condition support in policy engine

Add support for s3:ExistingObjectTag/<tag-key> condition keys in bucket
policies, allowing access control based on object tags.

Changes:
- Add ObjectEntry field to PolicyEvaluationArgs (entry.Extended metadata)
- Update EvaluateConditions to handle s3:ExistingObjectTag/<key> format
- Extract tag value from entry metadata using X-Amz-Tagging-<key> prefix

This enables policies like:
{
  "Condition": {
    "StringEquals": {
      "s3:ExistingObjectTag/status": ["public"]
    }
  }
}

Fixes: https://github.com/seaweedfs/seaweedfs/issues/7447

weed/s3api/policy_engine/engine.go

@@ -154,7 +154,7 @@ func (engine *PolicyEngine) evaluateStatement(stmt *CompiledStatement, args *Pol
 
 	// Check conditions
 	if len(stmt.Statement.Condition) > 0 {
-		if !EvaluateConditions(stmt.Statement.Condition, args.Conditions) {
+		if !EvaluateConditions(stmt.Statement.Condition, args.Conditions, args.ObjectEntry) {
 			return false
 		}
 	}