Introduce EVI proxy example

[twitchard]

Introduces an "EVI Proxy" example

Watch usage here: https://www.loom.com/share/2e7d7e110ba04d2ea3e26937c362441c

This example contains an EVI "proxy" that accepts a websocket connection from a client, connects to EVI, and forwards messages back and forth between the client and EVI.

This app is useful as an example in its own right: it demonstrates

• how to connect to EVI from a Typescript backend,
• how to accept websocket connections, process messages, and send them upstream to EVI

See upstream.ts and downstream.ts for more details.

It is also useful as a debugging tool: it supports

• recording and replaying EVI conversations,
• simulating error conditions that you might want to handle to make your EVI application more robust.