Enhance policy handling to support SSE and WORM (#5790)

- remove old bucket policy handling
- add new policy handling
- add new policy handling unit tests

This patch brings support to bucket policy to have more control not
limiting to anonymous.  Bucket owner controls to allow/deny any rest
API.

For example server side encryption can be controlled by allowing
PUT/GET objects with encryptions including bucket owner.

cmd/gateway/nas/gateway-nas.go

@@ -20,9 +20,9 @@ import (
 	"context"
 
 	"github.com/minio/cli"
-	"github.com/minio/minio-go/pkg/policy"
 	minio "github.com/minio/minio/cmd"
 	"github.com/minio/minio/pkg/auth"
+	"github.com/minio/minio/pkg/policy"
 )
 
 const (
@@ -132,6 +132,6 @@ func (l *nasObjects) IsNotificationSupported() bool {
 }
 
 // GetBucketPolicy will get policy on bucket
-func (l *nasObjects) GetBucketPolicy(ctx context.Context, bucket string) (policy.BucketAccessPolicy, error) {
-	return minio.ReadBucketPolicy(bucket, l)
+func (l *nasObjects) GetBucketPolicy(ctx context.Context, bucket string) (*policy.Policy, error) {
+	return minio.GetPolicyConfig(l, bucket)
 }