mod_rest: Support parameters in callback URL
E.g.
rest_callback_url = "http://myapi.example:5000/api/{kind}/{type}"
which results in e.g. requests to /api/message/chat
Allows using path routing in web frameworks for dispatch instead of
having to pick apart the payload to further dispatch it.
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summary: Allow server administrators to probe any user
...
This module lets server administrators send `<presence type="probe"/>`
to any local user and receive their presence in response, bypassing
roster checks.