[FEA] Add DiskANN Search support

[bkarsin]

Add DiskANN search functionality for the current Vamana build. Already have a GreedySearch kernel as part of build, but need to edit, optimize, and possibly add more options.

If possible, would be good to bring in ideas / improvements from the BANG paper work: https://arxiv.org/abs/2401.11324.

[cjnolet]

@bkarsin the current plan here is to bring in BANG for this, and we have discussed with the authors of BANG that the shortest path to initial capability is to add the BANG repository as a (build-time) dependency and invoking it directly to start, while we work to improve the BANG code itself (eg. using RMM, etc...) to contribute back upstream. The BANG authors would like to keep this separation, but in the meantime we can make our changes to the BANG code in the form of a git branch and patch it directly in cuVS while we wait to upstream it.

cc @tarang-jain

[bkarsin]

Ok, that sounds good. Would it eventually be incorporated directly into the cuvs codebase or always be a build-time dependency on a branch of their repo?