Follow up tasks to initial port of NN-Descent

[divyegala]

Following merge of #1748, there are some follow-up tasks that need to be performed to promote nn-descent from experimental namespace which are detailed here:

• Better support for pinned memory [FEA] Introduce universal_mdspan/mdarray vocabulary to RAFT #1819
• Add generalized bloom_filter
• Check existing bitonic_sort compatability
• Replace input data cast to __half and l2-norm calculation with RAFT utilities Port NN-descent algorithm to use in cagra::build() #1748 (comment)
• Investigate expanding number of vectors that can be processed by NN-Descent (current limit: std::numeric_limits<int>::max() / 2
• knn-graph built by nn-descent is currently in host-memory, but should have the ability to be in host or device memory as requested by the user Port NN-descent algorithm to use in cagra::build() #1748 (comment)
• Add NN-Descent as a top-level ANN algorithm (missing functionalities: search, extend)