Support for PKCE (Proof Key for Code Exchange) in openid provider

1. Proposed title of this feature request

Support for PKCE in our internal client openid provider.

2. What is the nature and description of the request?

Customer is using PKCE enforcement, like for instance, described in:

https://datatracker.ietf.org/doc/html/rfc7636

or in this implementation:

https://docs.pingidentity.com/r/en-us/pingone/p1_pkce_enforcement

3. Why does the customer need this? (List the business requirements here)

it's a security enforcement.

4. List any affected packages or components.

oauth openid client. It needs to support the code challenge / code verifier.