Improves performance of deserialization of vector data for calculating similarity functions.
Instead of deserializing vector data into a std::vector<data_value>, we deserialize directly into a std::vector<float>
and then pass it to similarity functions as a std::span<const float>.
This avoids overhead of data_value allocations and conversions.
Example QPS of `SELECT id, similarity_cosine({vector<float, 1536>}, {vector<float, 1536>}) ...`:
client concurrency 1: before: ~135 QPS, after: ~1005 QPS
client concurrency 20: before: ~280 QPS, after: ~2097 QPS
Measured using https://github.com/zilliztech/VectorDBBench (modified to call above query without ANN search)
Fixes https://scylladb.atlassian.net/browse/SCYLLADB-471Closesscylladb/scylladb#28615
668d6fe019