[FEA] Migrate nn-descent updates from RAFT

[cjnolet]

Recently, there were several updates made to the nn-descent algorithm in RAFT which need to be migrated over to cuVS. This includes:

1. Ability to compute mutual reachability distance by applying an epilogue during the distance computation.
2. Batching API for scalability
3. Ability to specify whether distances should be returned with search results (this option should ultimately be made available to all index types)

There might be more updates. @divyegala please keep me honest here.