Run Middleware within WorkStore (#70808)
The data structure inside WorkStore (formerly StaticGenerationStore) is
a mess with a bunch of options (what hidden class?). It's also there to
support Route Handlers, App Router Pages and Actions. It should probably
be a more specific disjoint union for each use case rather than just
optional fields.
We could have a separate kind of store for Middleware but it seems like
we're going towards Middleware becoming something with same semantics as
an App Route Handler. For example, we already support `cookies()`,
`headers()`, `after()`, `revalidate` etc. That's why it already has a
RequestStore.
This becomes a requirement for [moving `AfterContext` context to
`WorkStore`](https://github.com/vercel/next.js/pull/70806).
