[Segment Cache] Support dynamic head prefetching (#81677)
Fixes an issue where opting into dynamic prefetching with
prefetch={true} would not apply to head data (like the title), only the
page data. Although the head was being sent by the server as part of the
prefetch response, it wasn't being transferred correctly to the prefetch
cache.
The net result is that you can now fully prefetch a page with a dynamic
title without any additional network requests on navigation.
