Implement the "custom signature function" feature in the import instance of the webrunner.
It overwrites the "post-notification-run" script created by the resilient stack, in order to use the custom function defined by instances inside the webrunner, if it exists.
It re-uses the same code as for the exporter, thanks to https://lab.nexedi.com/nexedi/slapos.toolbox/merge_requests/45