Cache the head separately from the route tree (#84724)
Currently the head (i.e. the viewport and the metadata) is stored in the
same cache entry as the route tree. This is not ideal because the head
may or may not contain application data, and the response can differ
depending on the prefetch strategy. We need to be able to re-fetch the
head without also invalidating the route tree.
This refactors the Segment Cache to treat the head data as a special
kind of "segment". It doesn't appear in the route tree, so there's some
logic to account for that, but for the most part it reuses all the
existing implementation for fetching and caching normal segment data.
