[Experimental] Implement dataset as a single type+specs

[achirkin]

An experimental implementation draft of a dataset that keeps its main functionality in a single struct template.
It's parameterized by the value type, index type and the specification. The specification defines all return types and implementations for member functions. For example, the data_view() member returns an appropriate raft::mdspan for all dense datasets, but a sparse matrix view types for COO/CSR (sparse matrices).

The specs included in the example:

• empty_spec: has nothing but the dimension of the dataset
• mdarray_spec: implement plain (contiguous) and padded datasets via appropriate mdarrays.
• sparse_spec: datasets backed by sparse (COO/CSR) matrices from raft
• sq_spec: scalar compression
• vpq_spec: PQ compression plus centroids (composed dictionary)

[copy-pr-bot]

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