refactor(types): ensure compatibility with Express middlewares

In order to prevent issues like:

> error TS2345: Argument of type 'RequestHandler<ParamsDictionary, any, any, ParsedQs, Record<string, any>>' is not assignable to parameter of type 'Middleware'.
>  Types of parameters 'req' and 'req' are incompatible.
>  Type 'IncomingMessage' is missing the following properties from type 'Request<ParamsDictionary, any, any, ParsedQs, Record<string, any>>': get, header, accepts, acceptsCharsets, and 29 more.
>
>  io.engine.use(sessionMiddleware);
                 ~~~~~~~~~~~~~~~~~

Related: socketio/socket.io#4644

We could also have use the RequestHandler type from the
@types/express-serve-static-core package, but that would add 5 new
dependencies.

See also: https://github.com/socketio/engine.io/issues/673

lib/server.ts

@@ -320,7 +320,7 @@ export abstract class BaseServer extends EventEmitter {
    *
    * @param fn
    */
-  public use(fn: Middleware) {
+  public use(fn: any) {
     this.middlewares.push(fn);
   }