[PLAT-11373] Add support for deferring view load span end

[kstenerud]

Goal

Add defer APIs to allow the user to defer the end of the overarching view load span based on a condition:

    /**
     * Defer ending the overarching view load span until the supplied function returns true during a body build cycle.
     * The view load span will not be ended until ALL deferred-end conditions are true.
     */
    func bugsnagDeferEndUntil(deferUntil: @escaping ()->(Bool)) -> some View

    /**
     * Defer ending the overarching view load span until this view disappears from the view hierarchy.
     * The view load span will not be ended until ALL deferred-end conditions are true.
     */
    func bugsnagDeferEndUntilViewDisappears() -> some View

Design

Views can now have their overarching view load span end deferred like so:

struct SomeView: View {
    @State var data: Data?
    @State var deferringViewLoadSpan = true

    var body: some View {

        // Fake a background data load.
        // On iOS 15+ you'd use a .task()
        defer {
            DispatchQueue.global().async {
                data = Data()
            }
        }

        return VStack {
            if data == nil {
                // .bugsnagDeferEndUntilViewDisappears() will hold the current
                // view load span open until this Text view disappears.
                //
                // When this Text view disappears, its defer is resolved.
                // That would in theory leave the view load span free to end,
                // but there are still other defers to resolve!
                Text("SwiftUI is loading")
                .bugsnagTraced("loading")
                .bugsnagDeferEndUntilViewDisappears()
            } else {
                Text("Hello from SwiftUI 🙃")
                .bugsnagTraced("loaded")

                if deferringViewLoadSpan {
                    // Defer the view load span end until this button disappears.
                    Button("Stop deferring the view load span") {
                        deferringViewLoadSpan = false
                    }
                    .bugsnagDeferEndUntilViewDisappears()
                }
            }
        }
        .bugsnagDeferEndUntil {
            // Defer the view load span end until this function returns a true value.
            //
            // This is technically redundant since the above button is deferring the
            // view load span based on the same @State value (deferringViewLoadSpan).
            return !deferringViewLoadSpan
        }
    }
}

Testing

Added e2e tests

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