Check hashed passphrase returned by LCPAuthenticating

[mickael-menu]

When the test-app only had a hashed version of a passphrase, it could not send it through LCPAuthenticating.

This PR allows to provide both hashed and clear passphrases.

With this, it would be possible to implement retrieving the passphrases using Authentication for OPDS (as defined here: https://readium.org/lcp-specs/notes/lcp-key-retrieval.html) by creating a OPDSLCPAuthentication class implementing LCPAuthenticating.

You can also create a simple LCPAuthenticating implementation to provide a raw clean or hashed passphrase. The fallback is used, for example, to revert on showing a credentials pop-up if the passphrase was wrong.

final class LCPPassphrase: LCPAuthenticating {
    private let passphrase: String
    private let fallback: LCPAuthenticating
   
    init(_ passphrase: String, fallback: LCPAuthenticating) {
      self.passphrase = passphrase
      self.fallback = fallback
    }
   
    func requestPassphrase(for license: LCPAuthenticatedLicense, reason: LCPAuthenticationReason, completion: @escaping (String?) -> Void) {
        guard reason == .passphraseNotFound else{
            fallback.requestPassphrase(for: license, reason: reason, completion: completion)
            return
        }
     
        completion(self.passphrase)
    }
}