macOS: use SecItem API

[gaborcsardi]

Probably. Need to check if it has everything that we need. It will allow us to use iCloud keychains.

Some docs:
https://developer.apple.com/documentation/technotes/tn3137-on-mac-keychains
https://forums.developer.apple.com/forums/thread/724023
https://developer.apple.com/forums/thread/724013