Add Segment Cache feature check to `prefetch` API (#72861)
Wraps the `prefetch` API in a feature check for the Segment Cache, and
forwards the call to a stub for the new implementation.
Unlike the old implementation, the Segment Cache doesn't store its data
in the router reducer state; it writes into a global mutable cache. So
we don't need to dispatch a router action.
Since the Segment Cache isn't actually implemented yet, this effectively
disables prefetching when the experimental flag is enabled.
There's some validation that we do for prefetch URLs that I extracted
into a shared function. (For example, we only prefetch same-origin URLs,
and we don't prefetch anything in development.)
