Fix pkce bugs

[angelakis]

This PR aims to fix the issues outlined in #93.

[rohe]

In add_on/pkce I'd rather check for the authorization/token endpoints first and then do post_parse_request.
Along the line:

auth_endpoint =  endpoint.get("authorization")
if auth_endpoint is None:
    LOGGER.warning("No authorization endpoint found, skipping PKCE configuration")
    return 
else:
    auth_endpoint.post_parse_request.append(post_authn_parse)

[angelakis]

In add_on/pkce I'd rather check for the authorization/token endpoints first and then do post_parse_request.
Along the line:

auth_endpoint =  endpoint.get("authorization")
if auth_endpoint is None:
    LOGGER.warning("No authorization endpoint found, skipping PKCE configuration")
    return 
else:
    auth_endpoint.post_parse_request.append(post_authn_parse)

I agree. I put both checks before everything else.