Support synchronize through iCloud

Hi. Thank you for the wonderful library.

This library is not supported to synchronize through iCloud. I think there is demand.

To support iCloud, [kSecAttrSynchronizable](https://developer.apple.com/documentation/security/ksecattrsynchronizable) must be configurable. This is not so difficult. If this proposal is good, I will try it.

What do you think about this proposal?