Make webhook url an optional parameter in the request

[hairylime]

Configuration is cleaner when the request can include the callback url - also supports case where different classes / types of documents originate from different places and thus need multiple callback urls...

[hairylime]

Digging into the code and see the webhook config - alternate option for this would be allow multiple configured webhooks and thus the request could reference the configuration to use (so that secrets can still be required / used) - still has the problem of muddy configuration, but allows for different origins