[dynamicIO] model pathname access in metadata as async (#79136)
reading the pathname is akin to reading params and it should be an async
function so metadata can postpone during prerendering just like usercode
can.
Unforutnately the current metadata merging implementation was
synchronous. It might be tempting to just await the pathname high up and
let the merge continue to be sync but this might trigger dynamic
metadata when not necessary since only certain metadata properties
require the pathname. In an effort to maximize which metadata can be
prerendered I converted the necessary merging functions to be async and
passed a promise in that will exhibit the correct semantics when
accessed when dynamicIO is enabled.
