Can't set response headers in Server.handleUpgrade

Describe the bug
It's not possible to specify headers for an upgrade response that is handled by Server.handleUpgrade.

To Reproduce
Follow the usage example where requests are passed in explicitly:
https://github.com/socketio/socket.io/blob/439a8f669c6788b512dd76011c4476830d67515b/packages/engine.io/README.md?plain=1#L40-L58

Using this example it's not possible to specify the upgrade response headers. Specifically, it's not possible to set the CORS headers without using the .cors "middleware" option. Since https://github.com/socketio/socket.io/commit/d1f5aa93722a7f1ed729b96f771daf92a3dfdaf7 using the .cors "middleware" option breaks webtransport, so the only way to make it work is to remove the .cors option and set them up manually.

Expected behavior
It should be possible to set any headers for the upgrade response that is sent out when upgrade is handled by engine.io.

Engine.IO currently uses the kResponseHeaders key on the request object to specify additional headers to send in the upgrade response:
https://github.com/socketio/socket.io/blob/439a8f669c6788b512dd76011c4476830d67515b/packages/engine.io/lib/server.ts#L713-L732

I expected it to be possible to set them up from my code in a similar way:

const kResponseHeaders = Symbol.for("responseHeaders");

server.on("upgrade", (req, socket, head) => {
    if (req.url?.startsWith("/engine.io/")) {
        req[kResponseHeaders] = {
            "Access-Control-Allow-Origin": req.headers["origin"] || "*",
            "Access-Control-Allow-Credentials": "true"
        };
        return eio.handleUpgrade(req, socket, head);
    }
    // ...
});

But it is currently defined as a unique Symbol, which prevents above code from working:
https://github.com/socketio/socket.io/blob/439a8f669c6788b512dd76011c4476830d67515b/packages/engine.io/lib/server.ts#L27

The new definition can use Symbol.for instead to allow user code to specify extra headers for the websocket upgrade response.

Start in packages/engine.io/lib/server.ts at the unique kResponseHeaders definition and the Server.handleUpgrade response-header handling around lines 713–732. Compare this with the explicit-request usage in the engine.io README; done means user code can provide additional upgrade response headers and the documented CORS example works without the cors middleware.