PKCE issues

[angelakis]

We have found two (corner case) issues with PKCE:

• In configurations with PKCE enabled as not essential, and plain challenge method not supported, authentication requests without PKCE parameters fail. The reason behind this is that because the challenge method is missing, plain is set and afterwards a check for the code_challenge_method validity fails, because we do not support plain. I suppose that this check must be done only in case code_challenge is included in the request.

• add_pkce_support method assumes both authorization and token endpoints are configured so it makes no checks before trying to add PKCE hook methods. As a result it crashes if either is missing. I propose that instead of crashing we make the required checks and in the case either is missing, we log a warning and skip the configuration.