Track search string as part of "refresh state" (#87203)
When a parallel route does not match the current URL, in its place we
render the render the last active route that matched that slot. When we
next refresh the page, we track some additional state to inform the
router how to refresh the old, "inactive" routes that aren't reachable
from the outermost, "active" route.
Before this PR, the only state we tracked was the URL of the page that
last matched the inactive segment. After this PR, I am also tracking the
search query string of the old route. This can't be inferred from the
URL alone because the URL may have been rewritten by the server.
We don't need the search query to perform a network request — that is
handled by the URL alone — but we do need it in order to construct a
cache key for the page segments. Today we read the cache key from the
FlightRouterState, which has all the params embedded inside of it, but
eventually I want to remove all uses of FlightRouterState that aren't
about sending/receiving data from the server or storing the tree as
state in the browser's history entry. So this is an incremental step
towards that goal.
