feat: support configurable PKCE method

Objective

Allow users to configure the PKCE code challenge method used by the provider.

Motivation

Currently, the provider hardcodes the PKCE method to S256. While this is the most secure and common method, some OAuth 2.0 servers or specific security policies might require or only support other methods (e.g., plain). Adding support for a configurable PKCE method increases the providers flexibility and compatibility.

Todos

• Add a pkceMethod option to the AuthentikProvider constructor.
• Modify getPkceMethod() to return the configured method.
• Add unit tests to verify the new configuration option.
• Update README.md to document the pkceMethod option.