feat: support backend property on both Request and Response, as a Backend instance

[guybedford]

This updates req and res objects to provide a backend property lookup that now returns a Backend instance instead of a string.

This can be useful for checking the backend information a response was taken from, eg:

const res = await fetch('https://site.com/blah');
console.log(res.backend.isSSL);

etc. etc. with all the new backend options.