Need better separation of HTTP interface and WebAuthn functions

[JetForMe]

The core functions

• WebAuthnManager.beginRegistration()
• WebAuthnManager.finishRegistration()
• WebAuthnManager.beginAuthentication()
• WebAuthnManager.finishAuthentication()

should accept and return have native Swift types, rather than encoded types. For example, finishRegistration() takes a RegistrationCredential, which has a URLEncodedBase64 type for id. RegistrationCredential is also used directly for encoding/decoding in transit from the client.

This makes translating webauthn requests a lot more tedious or forces a very specific exchange between client and server.

The webauthn library probably shouldn't have any Codable classes in its core.