feat: add response header overrides for GetObject

GetObject allows overriding response headers with the following
paramters:
response-cache-control
response-content-disposition
response-content-encoding
response-content-language
response-content-type
response-expires

This is only valid for signed (and pre-singed) requests. An error
is returned for anonymous requests if these are set.

More info on the GetObject overrides can be found in the GetObject
API reference.

This also clarifies the naming of the AccessOptions IsPublicBucket
to IsPublicRequest to indicate this is a public access request
and not just accessing a bucket that allows public access.

Fixes #1501

auth/access-control.go

@@ -70,20 +70,20 @@ func VerifyObjectCopyAccess(ctx context.Context, be backend.Backend, copySource
 }
 
 type AccessOptions struct {
-	Acl            ACL
-	AclPermission  Permission
-	IsRoot         bool
-	Acc            Account
-	Bucket         string
-	Object         string
-	Action         Action
-	Readonly       bool
-	IsBucketPublic bool
+	Acl             ACL
+	AclPermission   Permission
+	IsRoot          bool
+	Acc             Account
+	Bucket          string
+	Object          string
+	Action          Action
+	Readonly        bool
+	IsPublicRequest bool
 }
 
 func VerifyAccess(ctx context.Context, be backend.Backend, opts AccessOptions) error {
-	// Skip the access check for public buckets
-	if opts.IsBucketPublic {
+	// Skip the access check for public bucket requests
+	if opts.IsPublicRequest {
 		return nil
 	}
 	if opts.Readonly {