Add enum UserDefaultsKey to UserDefaultsCore

[uhooi]

Managing UserDefaults keys together makes it easier to see what you are storing.

[uhooi]

public enum UserDefaultsKey: String {
  case foo
  // ...
}

[uhooi]

Change UserDefaultsClient from struct to class.

[uhooi]

ref: https://twitter.com/the_uhooi/status/1640858414711738369