feat: support server-sent events

[kettanaito]

• Closes First-class Server-Sent Events (SSE) API #2117

Todo

• Accept an OutgoingEvents type argument at sse() to make type-safe event property on the mock payload.
• Support passthrough(). How do you intercept an EventSource, interact with it from the mock, and then passthrough after that?
• Support event augmentation. How do you listen to the actual server's events, modify them, and send them back to the client?
• Add test for withCredentials affecting the request cookies propagation.
• How to handle passthrough()? What if I want to let the EventSource receive whichever events the actual server sends from now on? Still manual?
• Add tests for .use() with sse().
• Support retry
• Add an invariant for typeof EventSource !== 'undefined' to let the developer know when they are using sse() in the environment that doesn't support it.
• Documentation

References

• https://github.com/Azure/fetch-event-source

[kettanaito]

Augmenting actual server-sent events

sse(url, ({ source, server }) => {
  // This creates a new EventSource request
  // and propagates all the events from here
  // to the underlying pending stream for `source`.
  server.connect()
})

Modifying the event is similar to that in WebSockets: prevent its default and send a new event:

sse(url, ({ source, server }) => {
  server.connect()

  server.addEventListener('message', (event) => {
    // Prevent this server event from reaching the client.
    event.preventDefault()
    const newEvent = modify(event.data)
    source.send(newEvent)
  })
})

May be a good idea to rename source to client and have a consistent SSE/WebSocket experience.

[kettanaito]

The tests are failing likely due to Node.js bump to v20 in CI.

Edit: Looks like an issue specific to a particular version of Node.js. Reported here: nodejs/undici#3676