feat: add PasskeyAuthenticatorInterface implemented by PasskeyAuthenticator

[spydon]

What

Adds a PasskeyAuthenticatorInterface to passkeys_platform_interface and makes the passkeys package's PasskeyAuthenticator implement it.

// passkeys_platform_interface/lib/passkey_authenticator_interface.dart
abstract interface class PasskeyAuthenticatorInterface {
  Future<RegisterResponseType> register(RegisterRequestType request);
  Future<AuthenticateResponseType> authenticate(AuthenticateRequestType request);
}

// passkeys/lib/authenticator.dart
class PasskeyAuthenticator implements PasskeyAuthenticatorInterface { ... }

The interface is re-exported from passkeys_platform_interface.dart.

Why

Packages that integrate passkeys (for example Supabase's supabase_flutter) often only need to hand a RegisterRequestType/AuthenticateRequestType to an authenticator and read back the response. To do that today they must depend on the full passkeys plugin, which forces every consumer of their package to raise minimum platform versions (iOS 16.0 / macOS 13.5) and pull in the plugin's transitive dependencies, even when passkeys are never used.

With this interface, such a package can depend only on passkeys_platform_interface and accept a PasskeyAuthenticatorInterface. Apps that want passkeys add the passkeys plugin themselves and pass their PasskeyAuthenticator straight in; apps that do not are unaffected.

Non-breaking

PasskeyAuthenticator.register and .authenticate already have exactly the signatures the interface declares, so implements only adds a supertype. No method, behavior, or public-API changes.

Notes

• Versions, CHANGELOGs, and the intra-workspace passkeys_platform_interface constraint bump are left to the Melos release tooling.
• flutter analyze passes for both packages (the implements clause resolves cleanly, confirming the class satisfies the interface). The only analyzer output is pre-existing info-level lints in unrelated files.

Happy to adjust naming, file location, or whether the interface should also be re-exported from passkeys/types.dart to match your preferences.