Investigate providing a way for users to wrap custom native views

We could e.g. have nodes that represent opaque pointers to native views (can we safely convert them to integers, serialize them as JSON and convert them back? Otherwise we may have to pass them through FFI via a separate parameter and use a custom indexing to reference them in the node tree.).

Aside from the question of representation, we would have to design a nice API around it, ideally letting users provide different views for different backends. Perhaps conditional compilation against nuit's features could work (not sure if downstream library consumers can check for dependency features), but we would still want to handle the case where nuit was e.g. built with multiple backends correctly. That is, the user's app should support native views for every enabled backend.