Unify `setup_required` middlewares.

The two middlewares were identical but for the specific `IntoResponse` impl used to generate the response when setup has not been completed. However, unifying them while still using `from_fn_with_state` lead to this horrorshow:

    .route_layer(middleware::from_fn_with_state(
        app.clone(),
        |state, req, next| {
            setup::middeware::setup_required(UNAVAILABLE, state, req, next)
        }
    ))

It's a lot to read, and it surfaces the entire signature of a state-driven middleware `fn` into the call site solely to close over one argument (`UNAVAILABLE`).

Rather than doing that, I've converted this middleware into a full blown Tower middleware, following <https://docs.rs/axum/latest/axum/middleware/index.html#towerservice-and-pinboxdyn-future>. I considered taking this further and implementing a custom future to remove the allocation for `Box::pin`, but honestly, that allocation isn't hurting anyone and this code already got long enough in the translation.

The new API looks like:

    .route_layer(setup::Required::or_unavailable(app.clone()))

Or like:

    .route_layer(setup::Required::with_fallback(app.clone(), RESPONSE))

One thing I would have liked to have avoided is the additional `app.clone()` argument, but there isn't a way to extract the _state_ from a request inside of an Axum middleware. It has to be passed in externally - that's what `from_fn_with_state` is doing under the hood, as well. Using `State` as an extractor doesn't work; the `State` extractor is special in a _bunch_ of ways, and this is one of them. Other extractors would work. Realistically, I'd probably want to explore interfaces like

    .route_layer(setup::Required(app).or_unavailable())

 or

    .route_layer(app.setup().required().or_unavailable())

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