OAuth 2.1 compatibility

OAuth 2.1 compatibility:
- client_secret_post scheme is now default
- token endpoint authentication now possible with PKCE

Predefined defaults when adding new OAuth 2 client:
- confidential OAuth 2.1 : PKCE + token endpoint authentication
- public OAuth 2.1 : PKCE without token endpoint authentication
- confidential OAuth 2.0 : authorization code + token endpoint authentication
- public OAuth 2.1 : PKCE without token endpoint authentication