[CUBVEC-165] Replace unordered_set with ankerl::unordered_dense::set

[hgryoo]

http://jira.cubrid.org/browse/CUBVEC-165

Purpose

During HNSW index construction on the nytimes-256-angular dataset, flamegraph analysis revealed a significant performance bottleneck in the std::unordered_set used to track visited nodes during neighbor traversal.
The overhead originates from frequent hash operations, poor cache locality, and allocator pressure in this hot path.

This PR aims to reduce the traversal overhead by replacing the STL hash set with a higher-performance alternative better library

Implementation

• Added the ankerl hash container library to 3rdparty.
• Replaced the std::unordered_set used for visited_nodes in HNSW neighbor exploration with ankerl::unordered_dense::set.

Remarks

N/A