Passing a callback url to DIH

To provide an easy way to check if an DIH import process is completed (i.e., without having to implement a custom Solr EventListener), it should be possible to pass a callback url along a URL to a DIH request handler.

Ideally, the callback should somehow indicate the import operation's result (successful / not successful). This would obviate an additional call of the request handler to get the import operation's result and statistics, respectively.