Describe the feature

Review and implement OAuth 2.1 features: https://datatracker.ietf.org/doc/html/draft-ietf-oauth-v2-1-09

Additional context

Some features that are required in OAuth 2.1 are already implemented in OAuth2Provider (PKCE, PAR) as they're already in use by services as (optional) extensions to OAuth2. While OAuth 2.1 should be backwards compatible, new functionality should not bloat the OAuth2Provider class any further but instead go in OAuth21Provider that extends it, along with a new interface.

Are you (the requester) willing to submit a pull request for that feature?

[YES] (If I must!?)