Response serialization should support transports and authenticatorAttachment

[AlfioEmanueleFresta]

The WebAuthnIDLResponse::to_inner_model() implementations for MakeCredentialResponse and Assertion currently cannot populate two fields that are known at the transport layer:

1. transports (MakeCredential only) - hardcoded to Vec::new() in to_inner_model()
2. authenticator_attachment - hardcoded to None for both response types

Current workaround

Consumers (like credentialsd) must modify the returned RegistrationResponseJSON / AuthenticationResponseJSON after calling to_inner_model():

let mut registration_json = response.ctap.to_inner_model(request)?;
registration_json.response.transports = transports_from_transport_layer.clone();
registration_json.authenticator_attachment = Some(attachment_modality.clone());

Proposed Solution

• libwebauthn already has a concept of transports (AuthenticatorTransport enum)
• The transport implementations (UsbTransport, NfcTransport, etc.) could report their transport type, and authenticator_attachment could be derived from the authenticator info.
• The attachment modality ("platform" vs "cross-platform") can be determined based on the transport used
• This information is available during the WebAuthn operation but not currently captured in the response

References

• WebAuthn Level 3 spec: AuthenticatorAttestationResponseJSON.transports
• WebAuthn Level 3 spec: PublicKeyCredentialJSON.authenticatorAttachment