Initialize WebAuthn registration

Initialize a registration with Webauthn. Returns a JSON representation of CredentialCreationOptions for use with the Webauthn API’s navigator.credentials.create().

Note: The Webauthn API uses binary data represented by ArrayBuffers for certain input/output values. The Hanko API returns these values as base64url-encoded, so they must be converted to ArrayBuffers when passed to the Webauthn API. Similarly, Webauthn API output must be converted to base64url-encoded values when passed to the Hanko API (e.g. using the webauthn-json library).