Provide a SlicedModel DTO to render stable JSON representations of Slice

org.springframework.data.web.PagedModel was introduced in 3.3 via #3024 as a middle ground between serializing PageImpl directly and adopting Spring HATEOAS. There is currently no equivalent for Slice.

Problem

The rationale in #3024 applies equally to slices. SliceImpl is a domain type, so its JSON representation carries the same stability risk that motivated PagedModel — but returning a Slice from a controller today leaves three options, none good:

1. Serialize SliceImpl as-is, with the same instability PagedModel exists to avoid (and without the PlainPageSerializationWarning that flags the problem for pages).
2. Adopt Spring HATEOAS and SlicedResourcesAssembler (#1307), which is the documented answer but changes the payload to HAL.
3. Create a dedicated custom SlicedModel DTO per project.

Proposal

A SlicedModel<T> in org.springframework.data.web, mirroring PagedModel in shape and usage:

@GetMapping("/slice")
SlicedModel<?> slice(Pageable pageable) {
    return new SlicedModel<>(repository.findAllBy(pageable));
}

{
  "content": [ ... ],
  "page": { "size": 20, "number": 0, "numberOfElements": 20, "hasNext": true }
}

Deliberately no totalElements/totalPages, since that information is exactly what a Slice does not have.

Start with org.springframework.data.web.PagedModel and the Slice/SliceImpl types to understand the existing shape and usage. Add the corresponding stable SlicedModel representation without totalElements or totalPages, then verify its JSON shape against the example and existing PagedModel behavior.