Allow explicitly set identifiers on all systems

[antons]

I’m working on a suite of iOS and macOS apps that are going to share login information using iCloud Keychain. The apps will be distributed both on the App Store and directly, signed with Developer ID. As discussed in #140, switching between a directly distributed app and an App Store release shows a keychain prompt with the service identifier, so we have to use an explicitly set identifier. Since the items are synchronizable, we need to use the same explicitly set identifier on iOS. Currently Valet prevents this by restricting the explicitly set identifier APIs to macOS only. Would you be open to allowing the explicitly set identifier APIs on all systems? Ideally, I’d prefer to avoid maintaining a fork.