nginx: expose generated config and allow nginx reloads (#57429)

* nginx: expose generated config and allow nginx reloads

Fixes: https://github.com/NixOS/nixpkgs/issues/15906
Another try was done, but not yet merged in https://github.com/NixOS/nixpkgs/pull/24476

This add 2 new features: ability to review generated Nginx config
(and NixOS has sophisticated generation!) and reloading
of nginx on config changes. This preserves nginx restart on package
updates.

I've modified nginx test to use this new feature and check reload/restart
behavior.

* rename to enableReload

* add sleep(1) in ETag test (race condition) and rewrite rebuild-switch using `nesting.clone`