Decouple internal `data` usage for Bun websockets

[ClaudiuCeia]

What is the feature you are proposing?

Right now, implementing websockets with Bun and Hono feels like a real downgrade (see #3230 as well).
As @manastunga787 rightfully pointed out in that thread, the Bun websocket object is still available under ws.raw but:

• This is typed as optional. I'm not sure why, but of course it means peppering null checks wherever it's being used
• More importantly, the data attribute is being leveraged for internal use leading to unexpected errors onClose:

// ....

    return {
      onOpen: (event, ws) => {
        const { raw } = ws;
        if (!raw) {
          // is there a chance for this to ever be the case?
          return;
        }

        // this leads to
        // TypeError: undefined is not an object (evaluating 'websocketListeners.onClose')
        // because the data initially attached is being overwritten
        raw.data = {
          foo: "bar",
        };
      },
      // ...
  };

I'm now copying the previously attached data as well and everything seems to work fine, but it was rather painful to debug.
I'm wondering if data could be monkey-patched such that Hono specific data is not stored alongside application specific data.