Add struct/list support for load query options.

[jheer]

Discussed in #181

^{Originally posted by fundef1 September 21, 2023}
Hi, I'm trying to load a CSV and manually specify the columns.
it's a bit of a degenerate case as I only have a single column, but I can't figure out the correct format:

const csvopts = {
	auto_detect: false, 
	all_varchar: true,
	columns: "{'line': 'VARCHAR'}", 
	header: false
};

vg.coordinator().exec(vg.loadCSV("lines", `${window.location}data/lines.csv`,csvopts));

results in

Uncaught (in promise) Error: Parser Error: syntax error at or near "line"
LINE 1: ..._size=-1, all_varchar=true, columns='{'line': 'VARCHAR'}', header=false,...

the quoting of columns looks off and I've tried different variants of quoting the columns option but just can't get it to work.

any hints? thanks!

We should expand the load* methods to properly handle object (-> struct) and array (-> list) option values.