Automatically add all attributes for CLUSTER layers

[geographika]

Currently to add attributes to a layer with clustering you need to add a PROCESSING command e.g:

PROCESSING "ITEMS=Field1,Field2,Field3"

See supported processing options.

In order to reuse the same layer as a WFS it would be useful to have an option to set all fields automatically e.g.

PROCESSING "ITEMS=*" or PROCESSING "ITEMS=all" (similar to "gml_include_items" "all")

Otherwise the attributes have to be manually updated each time the DATA clause changes.

Function for reference.

Possibly related tickets #5219 #5292