Is your feature request related to a problem? Please describe.

socket.io (here engine.io) uses express's cors library for cors (dependency). I am currently developing a framework and had to write my own cors package in this framework. My framework supports websocket and uses socket.io. So when I try to use my cors package for websocket I do some very unpleasant things.

Describe the solution you'd like

The corsMiddleware function of the BaseServer class must be explicitly changed over the io instance or a middleware must be entered as a cors option.

Describe alternatives you've considered

Alternatively, the following option should be entered:

the current code is right here

    if (this.opts.corsMiddleware) {
       this.corsMiddleware = this.opts.corsMiddleware;
    } else if (this.opts.cors) {
      this.corsMiddleware = require("cors")(this.opts.cors);
    }

and here

    if (this.corsMiddleware) {
      const next = () => {
         this.verify(req, false, callback);
      }
      this.corsMiddleware(this.opts.cors)(req, res, next)
    } else {
      this.verify(req, false, callback);
    }

Although this is not a very good example, I think it is enough to explain my problem. I can create a better PR if you give ideas and support.

Best,
Sami Salih İbrahimbaş