fix(auth): make webauthn param optional and move register params to webauthn

[mandarini]

Moved from: supabase/auth-js#1125
Author: @Bewinxed

What kind of change does this PR introduce?

fix: make webauthn param optional in .authenticate() and .register() and move register params to webauthn: object for consistency

Bug fix / API improvement

What is the current behavior?

The webauthn parameter in the _authenticate() method is required, and the _register() method has WebAuthn-related parameters (rpId, rpOrigins, signal) at the top level instead of being grouped under a webauthn object.

What is the new behavior?

• Made the webauthn parameter optional in _authenticate() method with default empty object fallback
• Moved WebAuthn-related parameters in _register() method into a webauthn object for consistency with _authenticate()
• Both methods now have consistent API signatures with optional webauthn parameter

Additional context

None.