WebSocket client module

The KittyCAD Rust API client lets users connect to our WebSocket API. But OpenAPI can't tell users what the server expects to receive over that WebSocket, or what it intends to send over the WebSocket either.

This library solves the problem. It encapsulates the WebSocket connection from our API client, and adds methods to send and receive the correct types. This way users will always send the right types of requests and always receive the right types of responses.

Right now, this API sends over WebSocket text messages (JSON) to make it easier to debug with standard web debugging tools. But in the future, we can offer WebSocket binary messages (BSON), for faster de/encoding.