[MAJOR] Extract creating decorated engine to the separate function

[pawelfras]

          [MAJOR]

I guess we longer need the ngExpressEngine to be passed to some other function that decorates it, because now we control and create the ngExpressEngine. The decorator was meant to wrap the original Angular's ngExpressEngine that we don't create.

That said, removing it would be yet another migration to provide.

Example, but let's not do it now, only if we have more time:

import { createNgExpressEngine } from '@spartacus/setup/ssr;

const ngExpressEngine = createNgExpressEngine(ssrOptions);

Originally posted by @Platonn in #18197 (comment)