[FEA] Migrate remaining VSS / clustering APIs from RAFT

[cjnolet]

We've migrated over most of the important VSS/clustering APIs to cuVS but a few remain:

• Random ball cover (Corey)
• Sparse distances (Divye?)
• Sparse knn (Divye?)
• Spectral clustering
• Expose epilogue for bfknn (and others)
• Primitives benchmarks
• raft-ann-bench (Tamas & Artem)
• CAGRA expose build_knn_graph and optimize_graph helpers
• K-selection instantiations