feature: Support iCloud Keychain

[evandelaney]

This code change adds support for kSecAttrSynchronizable, which is iCloud Keychain syncing.

I'm not sure if you want to treat this the same way you've handled kSecAttrAccessGroup or if there's another design you had in mind. The reason I call this out is because kSecAttrAynchronizable allows for three values when querying: true (kCFBooleanTrue), false (kCFBooleanFalse), and kSecAttrSynchronizableAny (meaning "either").

Supporting iCloud Keychain in KVault helps my migration from a fully native iOS app to KMM. Furthermore, iCloud Keychain is a nice end user feature that preserves keychain items as users upgrade devices or pick up multiple devices.