fix: add new auth method to update ownership within acl

Add helper util auth.UpdateBucketACLOwner() that sets new
default ACL based on new owner and removes old bucket policy.

The ChangeBucketOwner() remains in the backend.Backend
interface in case there is ever a backend that needs to manage
ownership in some other way than with bucket ACLs. The arguments
are changing to clarify the updated owner. This will break any
plugins implementing the old interface. They should use the new
auth.UpdateBucketACLOwner() or implement the corresponding
change specific for the backend.

s3api/controllers/admin.go

@@ -15,13 +15,10 @@
 package controllers
 
 import (
-	"encoding/json"
 	"encoding/xml"
-	"fmt"
 	"net/http"
 	"strings"
 
-	"github.com/aws/aws-sdk-go-v2/service/s3/types"
 	"github.com/gofiber/fiber/v2"
 	"github.com/versity/versitygw/auth"
 	"github.com/versity/versitygw/backend"
@@ -172,27 +169,7 @@ func (c AdminController) ChangeBucketOwner(ctx *fiber.Ctx) error {
 			})
 	}
 
-	acl := auth.ACL{
-		Owner: owner,
-		Grantees: []auth.Grantee{
-			{
-				Permission: auth.PermissionFullControl,
-				Access:     owner,
-				Type:       types.TypeCanonicalUser,
-			},
-		},
-	}
-
-	aclParsed, err := json.Marshal(acl)
-	if err != nil {
-		return SendResponse(ctx, fmt.Errorf("failed to marshal the bucket acl: %w", err),
-			&MetaOpts{
-				Logger: c.l,
-				Action: metrics.ActionAdminChangeBucketOwner,
-			})
-	}
-
-	err = c.be.ChangeBucketOwner(ctx.Context(), bucket, aclParsed)
+	err = c.be.ChangeBucketOwner(ctx.Context(), bucket, owner)
 	return SendResponse(ctx, err,
 		&MetaOpts{
 			Logger: c.l,