Feature: Force delete / cascading delete

### Feature description

When deleting a pipeline it needs to be empty (i.e. no connectors or processors attached). Same goes for connectors. We can make it easier to delete a pipeline or connector by letting the user specify a "force" option which triggers a cascading delete of the entity and all entities attached to it. We need to figure out how to design the endpoint as DELETE endpoints should not contain a request body (relevant [SO question](https://stackoverflow.com/questions/14323716/restful-alternatives-to-delete-request-body)).