Allow defining a project workboard column trigger which sets a "dynamic" assignee, similar to the "Current User" functionality already existing in other "Assigned to" fields, for example Maniphest's Advanced Search form.
The logic is similar to PhabricatorPeopleNoOwnerDatasource; it defines const FUNCTION_TOKEN = 'viewer()' to avoid Value for "Assign task to" rule is invalid: User PHID ("viewer()") is not a valid user.
Note: In the meantime my user clarified that they do not only want every task to always be assigned in a column but are interested in only assigning tasks to the current user when the task is currently unassigned. That would be a separate followup patch obviously.
Closes T16058