mod_http_oauth2: Calculate client secret expiry in registration response Not actually the client_secret that expires, but the client_id JWT. Returning '0' indicating no expiry was incorrect unless JWT expiry is turned off, which we check for now.

This module helps make BOSH and WebSocket connection endpoints
discoverable via the HTTP method described in
[XEP-0156](https://xmpp.org/extensions/xep-0156.html#http).