Limit to 20 preloads

[twifkak]

The Google SXG cache has a limit of 20 preloads. I don't see anything in the webpackager code that guarantees that requirement is met. (Did I miss it?) We should provide a [default enabled] option to do so.

Seems especially necessary given the optional HTMLTask to promote preload tags to headers, where the former are more prominent today, with no such limit.

[yuizumi]

We don't have logic to limit the preloads to 20 (or any number), as far as I can recall. One easy approach is maybe to write a common processor to cut off Preloads (of exchange.Response) to the given limit, then register it as a postprocessor.

There's also a processor that propagates preload headers. (code)