feat(op): always verify code challenge when available (#721)

Finally the RFC Best Current Practice for OAuth 2.0 Security has been approved.

According to the RFC:

> Authorization servers MUST support PKCE [RFC7636].
> 
> If a client sends a valid PKCE code_challenge parameter in the authorization request, the authorization server MUST enforce the correct usage of code_verifier at the token endpoint.

Isn’t it time we strengthen PKCE support a bit more?

This PR updates the logic so that PKCE is always verified, even when the Auth Method is not "none".

example/server/exampleop/templates/login.html

@@ -25,5 +25,5 @@
             <button type="submit">Login</button>
         </form>
     </body>
-</html>`
-{{- end }}
+</html>
+{{- end }}