feat: add public memberwise initializers to passkey challenge types

[subhankarmaiti]

Summary

• Add public init to PasskeyEnrollmentChallenge, PasskeySignupChallenge, and PasskeyLoginChallenge
• These types previously relied on synthesized internal initializers, making them impossible to construct outside the module

Motivation

The React Native Auth0 SDK needs to reconstruct challenge objects on the native bridge side.

Without public initializers, consumers cannot create these structs directly, which blocks integration with wrappers and cross-platform SDKs that deserialize challenge data from JavaScript.

Testing

• swift build passes
• All existing tests continue to pass (swift test)
• Used in react-native-auth0 iOS bridge to verify enrollment flow end-to-end