Census cell similarity search: set TileDB-Vector-Search indexing parameters and characterize accuracy+perf

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The TileDB-Vector-Search indexing procedure has some settings we can tune. Consult with TileDB on a suitable parameter set for the Census cell embeddings, build indexes using chanzuckerberg/cellxgene-census#1112, and do some initial characterization of the search accuracy and performance. The accuracy and performance don't need to be perfect for now- this will be a starting point to iterate from.