feat: add the "initial_headers" and "headers" events

Those events will be emitted before the response headers are written to
the socket:

- "initial_headers": on the first request of the connection
- "headers": on all requests (HTTP long-polling and WebSocket upgrade)

Syntax:

```js

server.on("initial_headers", (headers, req) => {
  headers["test"] = "123";
  headers["set-cookie"] = "mycookie=456";
});

server.on("headers", (headers, req) => {
  headers["test"] = "789";
});
```

Related:

- https://github.com/socketio/engine.io/issues/557
- https://github.com/socketio/socket.io/issues/3630