wasm: create example which sets COOP and COEP from service worker

Multithreading and SharedArrayBuffer require cross-origin isolation, which can be enabled by setting the COOP and COEP headers. This can happen in two ways:

• On the server (e.g. 'qtwasmserver – --cross-origin-isolation', or by editing server config)
• Prom a PWA service worker, which can modify headers for http requests. This has the benefit that no server changes are needed.

Add service worker example to Qt Web Utils at git.qt.io/mosorvig/qt-web-utils