Support Jackson's new webhooks thing

[michielbdejong]

We can easily use https://github.com/o-development/solid-webhook-client to define a test suite for it.
Your post in CommunitySolidServer/CommunitySolidServer#1074 (comment) gives a nice overview of all the links to proposal texts

See https://gitter.im/solid/test-suite?at=61b9b0f24024f534f2bddb32