This adds a client for the interception proxy, and passes all test
WebSocket traffic through the proxy. It does not currently perform any
modifications to the WebSocket traffic (I’ll add a subsequent commit
where we convert a handful of tests to use the proxy instead of private
APIs for intercepting and modifying Realtime protocol messages).

This is very much a prototype and work in progress, hence all the TODOs.
The API of InterceptionProxyClient will probably evolve as we convert
more tests to use it.

I’ve also added an NPM script called test:playwright:open-browser
script, which lets you open a headed browser which is configured to use
the interception proxy. Useful for local debugging of browser tests.

Here’s how to run the tests locally on macOS:

One-time setup:

1. Install the interception proxy’s Python dependencies (see readme of
   [1])

2. Run `npm install` to install the interception proxy

3. Generate mitmproxy TLS certificate:

   `npx --package interception-proxy generate-mitmproxy-certs`

Running the tests:

1. In a separate terminal, run the interception proxy (after each test
   run, you’ll need to kill this and run it again, which is a bit
   annoying; it would be good to fix this):

   `npx interception-proxy`

2. Run the tests:

   - Node:

     `NODE_EXTRA_CA_CERTS=~/.mitmproxy/mitmproxy-ca-cert.pem npm run test:node`

   - Browser: The easiest one is Firefox since it doesn’t require you to
     install any CA certificates:

     `PLAYWRIGHT_BROWSER=firefox npm run test:playwright`

[1] https://github.com/ably-labs/interception-proxy

I’ve changed these tests to use the interception proxy instead of
private API for intercepting and injecting Realtime protocol messages,
as an example of how to use the proxy and its test suite client class.