Can't set response headers in Server.handleUpgrade

[weary-adventurer]

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:

socket.io/packages/engine.io/README.md

Lines 40 to 58 in 439a8f6

	#### (C) Passing in requests
	```js
	const engine = require('engine.io');
	const server = new engine.Server();
	server.on('connection', socket => {
	socket.send('hi');
	});
	// …
	httpServer.on('upgrade', (req, socket, head) => {
	server.handleUpgrade(req, socket, head);
	});
	httpServer.on('request', (req, res) => {
	server.handleRequest(req, res);
	});
	```

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 d1f5aa9 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:

socket.io/packages/engine.io/lib/server.ts

Lines 713 to 732 in 439a8f6

	if (typeof this.ws.on === "function") {
	this.ws.on("headers", (headersArray, req: EngineRequest) => {
	// note: 'ws' uses an array of headers, while Engine.IO uses an object (response.writeHead() accepts both formats)
	// we could also try to parse the array and then sync the values, but that will be error-prone
	const additionalHeaders = req[kResponseHeaders] || {};
	delete req[kResponseHeaders];
	const isInitialRequest = !req._query.sid;
	if (isInitialRequest) {
	this.emit("initial_headers", additionalHeaders, req);
	}
	this.emit("headers", additionalHeaders, req);
	debug("writing headers: %j", additionalHeaders);
	Object.keys(additionalHeaders).forEach((key) => {
	headersArray.push(`${key}: ${additionalHeaders[key]}`);
	});
	});
	}

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:

socket.io/packages/engine.io/lib/server.ts

Line 27 in 439a8f6

const kResponseHeaders = Symbol("responseHeaders");

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