template.uri
URI, possibly with :-prefixed parameter names. For example "/get?x=:x;y=:y" has parameters "x" and "y". (See dependent.params
next).
(Note: this follows the way of doing it in combobox.param). This is optional. If provided, then the front-end can use this callback to allow the user to
download a template. This is a template in two senses: the URI itself may be a template whose parameter values should be filled in, and the return value
of the request is an excel file which is a template for the user to fill in.