fix(util): make option_layer unify branch errors to BoxError

[ameyypawar]

Summary

Fixes #665.

option_layer returns Either<L, Identity>. The service-level Either<A, B> requires both branches to share an error type (B: Service<Request, Response = A::Response, Error = A::Error>). So when the optional layer changes the error type — e.g. TimeoutLayer, whose service errors are BoxError — over a service with a different error type, the result does not implement Service at all:

let svc = ServiceBuilder::new()
    .option_layer(Some(TimeoutLayer::new(Duration::from_secs(1))))
    .service_fn(|()| async { Ok::<_, io::Error>(()) }); // BoxError vs io::Error -> not a Service

The existing doc test didn't catch this because it never asserted that the result is a Service.

Why the fix has to box at the service level

A generic Layer<S> impl can't name S's error type — Layer has no Request parameter, so <S as Service<Request>>::Error isn't in scope when the layer is applied. The error types are only known once a request type is fixed, i.e. in the produced service's own Service impl. So the unification can't be done by composing MapErrLayer at option_layer time; it must live in a dedicated service.

The fix

Add OptionLayer<L> (the layer now returned by option_layer). Its produced OptionService<A, B> is like Either, but its Service impl sets Error = BoxError with bounds A::Error: Into<BoxError>, B::Error: Into<BoxError>, and B::Response = A::Response — so the layered and unlayered branches need not share an error type. ServiceBuilder::option_layer's return type is updated to match.

// before:  pub fn option_layer<L>(layer: Option<L>) -> Either<L, Identity>
// after:   pub fn option_layer<L>(layer: Option<L>) -> OptionLayer<L>

⚠️ Breaking change — and this is a proposal

This is semver-breaking and adds public API, so I'm raising it as a proposal rather than assuming the shape:

• Changed return types (breaking): util::option_layer (Either<L, Identity> → OptionLayer<L>) and ServiceBuilder::option_layer (Stack<Either<T, Identity>, L> → Stack<OptionLayer<T>, L>).
• New public items: util::OptionLayer, util::OptionService, and util::future::OptionResponseFuture.
• Names/placement are open to your preference (OptionService's Some/None variants, the OptionResponseFuture alias, etc.).
• An alternative discussed in Either rework broke option_layer #665 was impl Layer<S> for Option<L>; that has the same error-unification need, so the OptionService boxing machinery would still be required underneath. Happy to reshape toward whichever direction you prefer.

Tests / verification

• Adds a regression test (tests/util/option_layer.rs) that builds option_layer over a layer which changes the error type and asserts the result is a usable Service. It fails to compile before this change (the two Either branches have different error types) and passes after.
• Locally verified: cargo fmt --check, cargo check --workspace --all-features --all-targets, the new test, and the option_layer/ServiceBuilder::option_layer doc tests all pass; clippy and the all-features build are warning-clean. The change uses only match / Poll::map_err / Into::into, so it's within the current MSRV.