Add back ability to export pages from a CLI

We used to be able to use URLs with query string parameters to perform exports, see https://www.xwiki.org/xwiki/bin/view/Documentation/UserGuide/Features/Exports#HAdvanced-2

This was lost when moving to the PDF Export app (client side export) -> https://extensions.xwiki.org/xwiki/bin/view/Extension/PDF%20Export%20Application/

We need to have it back, probably through a REST API, to be able to perform exports from a CLI.