Add WebAuthn/Passkeys Support

[zoedsoupe]

Add WebAuthn/FIDO2 support for passwordless authentication with security keys, Touch ID, and Windows Hello. JS reference: auth-js/src/lib/WebAuthnClient.ts. Related to #54.

[zoedsoupe]

WebAuthn Status Update:

✅ Already Implemented (Server-Side Verification):

• WebAuthn MFA enrollment: Supabase.Auth.MFA.enroll(client, session, %{factor_type: :webauthn})
• Challenge creation: Supabase.Auth.MFA.challenge/4
• Credential verification: Supabase.Auth.MFA.verify/5

See: Supabase.Auth.MFA module and MFA Guide

❌ Not Needed in Elixir SDK:

• Browser credential creation (navigator.credentials.create()) - requires browser APIs
• Credential request (navigator.credentials.get()) - requires browser APIs

What This Issue Could Become:
If there's value, this could be about:

1. Helper utilities for parsing/preparing WebAuthn challenge options server-side
2. Documentation showing complete WebAuthn flow (client JS + server Elixir)

Otherwise, this can be closed as the core server-side verification is already complete in the MFA module.

@zoedsoupe - Should we close this as complete, or re-scope it for utilities/docs?