Fix client secret basic

[H2CK]

Improved handling of client_secret_basic authentication. But still this must be implemented using a pseudo user backend since the Nextcloud Core intercepts any requests containing an Authorization header.

• Improved pseudo user backend concerning integration with Nextcloud Core
• Corrected urldecoding for clientId and clientSecrets
• Allow to disable/enable client_secret_basic authentication in discovery endpoint using occ config:app:set oidc disable_auth_client_secret_basic --value true in case it should be disabled
• client_secret_basic is automatically disabled at discovery endpoint when system value ´token_auth_enforced´ is enabled. Since in this case the pseudo user backend will not be called by the Nextcloud Core.