Allow editors of the scenario block to select arguments from a list of checkboxes. The ones that are checked are added to the webhook's query parameters with their default name.

Possible arguments are defined using tagged services and a Public API interface. The service, when invoked, computes the value. It only returns strings (arrays are returned as comma separated values). If no value can be returned (anonyous user, no customer group...), some kind of empty or null value should be returned, and the query parameter gets an empty value.

We should by default support user_id and customer_group.