Expose a property to use PKCE with OpenID providers

Some OpenID providers re expecting a code_challenge but Nuxeo does not provide this information into the HTTP call. It produces this error in the OpenID logs

HTTP 302 Invalid request of ticket : code_challenge  

This is due to the fact that PKCE is not implemented into the Auth code flow generated by Nuxeo.

It could be done by calling the enablePKCE() method when building the AuthorizationCodeFlow object by the NuxeoOAuth2ServiceProvider#getAuthorizationCodeFlow

Ideally calling this method would be controlled by a new property on the corresponding description : OAuth2ServiceProviderDescriptor