Add `AuthenticationMethod` to `AuthenticateResponse`.

[dezlitz]

Description

The API endpoint /user_management/authenticate has been recently updated to include a field called authentication_method in the response.

This PR adds the new field to the AuthenticateResponse struct.

Documentation

Does this require changes to the WorkOS Docs? E.g. the API Reference or code snippets need updates.

[ ] Yes

If yes, link a related docs PR and add a docs maintainer as a reviewer. Their approval is required.