Improve prevent-fetch — add ability to set random response content

[AdamWr]

Similar to - https://github.com/AdguardTeam/Scriptlets/issues/199#issuecomment-1063271758 but for prevent-fetch.

Also adding Content-Length header to response would be a good idea, related to - https://github.com/AdguardTeam/AdguardFilters/issues/175797 and https://github.com/AdguardTeam/AdguardFilters/issues/176137

Steps to reproduce:

1. Add this rule:

   example.org#%#//scriptlet('prevent-fetch', 'pagead2.googlesyndication.com')

2. Go to - https://example.org/
3. In browser console run:

   (async () => {
   const ads = await fetch('https://pagead2.googlesyndication.com/pagead/js/adsbygoogle.js');
   const contentLength = (await ads.text()).length;
   const contentLengthHeader = Number(ads.headers.get('Content-Length'));
   if (!ads.ok || contentLength < 100 || contentLengthHeader < 100) {
       alert('Ads are blocked!');
   }
   })();

   Alert will be displayed.