allow CORS on webpack-dev-server (#11325)

* allow CORS on webpack-dev-server

* re-position header config on webpackDevServer config

Author: hasanayan

packages/react-scripts/config/webpackDevServer.config.js

@@ -44,6 +44,11 @@ module.exports = function (proxy, allowedHost) {
     // Note: ["localhost", ".localhost"] will support subdomains - but we might
     // want to allow setting the allowedHosts manually for more complex setups
     allowedHosts: disableFirewall ? 'all' : [allowedHost],
+    headers: {
+      'Access-Control-Allow-Origin': '*',
+      'Access-Control-Allow-Methods': '*',
+      'Access-Control-Allow-Headers': '*',
+    },
     // Enable gzip compression of generated files.
     compress: true,
     static: {