Refactor SdcController::preview() to use ComponentTreeHydrated, and update it to support nested components

@tedbow This should be a big accelerator to your work in 📌 Introduce an example set of representative SDC components; transition from "component list" to "component tree" Fixed — this will make your work there be visible in the UI.

IOW: this is the subset of #3446722 that interacts with the client directly, and requires server-side code to be updated for the client to be usable.

Review suggestion: review only ComponentTreeHydratedTest. If that part makes sense to you, IMHO you can blindly approve this, because it'll represent a leap forward with strict test coverage. That would unblock you on #3446722, and is an important step towards finalizing the client's data model and the request/response flow.

I just realized that the MR's scope goes a bit further than the originally described/intended scope. Updated issue summary to explain why.

Thanks @Wim Leers

This introduced a small regression: 🐛 First `preview` request fails to generate a response for empty layout Needs review .

Automatically closed - issue fixed for 2 weeks with no activity.