Values:
ACL_UNSPECIFIED (0):
ACL behavior is unspecified.
ACL_DESTINATION_BUCKET_DEFAULT (1):
Use the destination bucket's default object
ACLS, if applicable.
ACL_PRESERVE (2):
Preserve the object's original ACLs. This requires the
service account to have storage.objects.getIamPolicy
permission for the source object. Uniform bucket-level
access <https://cloud.google.com/storage/docs/uniform-bucket-level-access>__
must not be enabled on either the source or destination
buckets.