mod_http_oauth2: Declare additional client registration fields as strings Previously any property not listed in the schema was allowed in any form, which is probably a bit too liberal. Instead, limit extra fields to simple strings, which should still allow localized versions of the various URIs and descriptive properties per RFC 7591 §2.2

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).