Create endpoint that allows introspection in fields available for surcharges/restrictions

[brent-hoover]

Both our surcharges and restrictions capabilities allow you to apply "rules" which are field operator (like eq) value. When creating these in Kinetic we need to know which fields are available for use in these operators. Probably could use the SimpleSchema instances to introspect what fields are there and what value types they are. Probably good to investigate exactly how these plugins are implemented to see to exactly what things you can operate on (is it just product or is it anything in the cart)