implement "Upgrade" as a function in a api route to support websocket

Describe the problem

One can easy implement GET, PUT in an api route by a function (see below)

import { json } from '@sveltejs/kit';

export function GET() {
	const number = Math.floor(Math.random() * 6) + 1;

	return json(number);
}

Describe the proposed solution

If we implement UPGRADE Protocol_upgrade_mechanism we could implement websockets aswell

this file is in src/route/chat-app/server.js connect with new WebSocket('/chap-app')

// if this file is in src/route/chat-app/server.js

import { json } from '@sveltejs/kit';

// called when http call wants to upgrade to websocket protocol
export function UPGRADE() {

      // this function will be called after successfull upgrade
       return function connection(ws: WebSocket) {
                         
 	       ws.send('hello');
               ws.onmessage = (event) => {..});
              // other handlers
      }
}

### Alternatives considered

_No response_

### Importance

would make my life easier

### Additional Information

Currently, customers expect some kind of live real time interaction,  a clean solution integrating with sveltekit would be a huge advantage

Start with the proposed route example in src/route/chat-app/server.js and review the linked HTTP protocol upgrade documentation. Determine how an UPGRADE handler would fit the API route lifecycle and WebSocket connection handling. Done means a documented, supported way to establish and handle WebSocket connections through a SvelteKit route, with tests or examples showing the expected behavior.