Allow attaching multiple WebSocket proxies

[yoursunny]

Previously, if multiple WebSockets-enabled proxies are added to the same fastify.Server instance at distinct URI prefixes, the program crashes with error:

/app/node_modules/.pnpm/[email protected]/node_modules/ws/lib/websocket-server.js:363
      throw new Error(
      ^

Error: server.handleUpgrade() was called more than once with the same socket, possibly due to a misconfiguration
    at WebSocketServer.completeUpgrade (/app/node_modules/.pnpm/[email protected]/node_modules/ws/lib/websocket-server.js:363:13)
    at WebSocketServer.handleUpgrade (/app/node_modules/.pnpm/[email protected]/node_modules/ws/lib/websocket-server.js:339:10)
    at Server.upgrade (/app/node_modules/.pnpm/[email protected]/node_modules/ws/lib/websocket-server.js:114:16)
    at Server.emit (node:events:539:35)
    at onParserExecuteCommon (node:_http_server:732:14)
    at onParserExecute (node:_http_server:646:3)
 ELIFECYCLE  Command failed with exit code 1.

This PR fixes this problem by only attaching one WebSocket.Server to the same HTTP server, and then internally dispatches WebSocket connections to different upstreams via longest prefix match lookup among registered prefixes.

Checklist

• run npm run test
• npm run benchmark - it's unavailable in this repository
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct