Support selection cascades

[ChinHairSaintClair]

Due to config being captured in JSON format, and the current state of ValidatorSelectOne, it does not seem possible to filter selections such as through the XLSXForm.
Eg. Select a sub node, which filters available options for the nodes, then select a node from the list.

A small subset of our cascade (in the choices sheet) can see below:

list_name	name	label::en	sub_district
sub_districts_metro	eastern	Eastern
sub_districts_metro	khayelitsha	Khayelitsha
nodes	EERSTE_RIVER_CDC	EERSTE RIVER CDC	eastern
nodes	WESBANK_CDC	EERSTE RIVER CDC	eastern
nodes	TOWN_2_CDC	TOWN 2 CDC	khayelitsha
nodes	NOLUNGILE_CDC	NOLUNGILE CDC	khayelitsha

The form items are as follows:

type	name	label::en	choice_filter
select_one sub_districts_metro	sub_district	Sub District
select_one nodes	node	Node	sub_district=${sub_district}