[Segment Cache] Send <head> during route prefetch (#72890)
Based on:
- #72874
---
The <head> does not belong to any particular segment; it represents the
entire page. Except in the case where two URLs rewrite to the same
result, it's very unlikely to benefit from deduplication, so there's no
benefit to caching it separately from the route tree. So we'll send it
in the same response.
Since the head may contain dynamic data, the route tree response may now
contain hanging promises, so we'll need to use the same AbortController
trick that we use for the segments.
