Restructure PKCE handling

[maennchen]

This issue is here to track improvements whenever we start work on a new major release since it will cause breaking changes. It is not intended to be solved right now.

Current situation

PKCE will only be requested when the pkce_verifier option is passed.

Desired

PKCE will automatically be enabled & a secret generated if there's supported code challenges. The result of the authorization function should result in both a url and the PKCE verifier if one was used.