Websocket params should be a struct, not positional

[adamchalmers]

Currently the modeling API's websocket has a lot of parameters:

It's very annoying to update these, because each time we add an argument, the consumers have to look up the old order and the new order, and make sure all the arguments are correct.

IMO this would be much easier if the websocket endpoint accepted a single parameter of type struct ModelingWsParams with named fields.