Move disk-index PQ flat scan to new API. #1104

The repo has supported an API to perform brute force scan over flat index types in diskann::flat.

We are trying to port all brute force search implementations in the repo to this framework. The only major one left is the flat scan implementation in the disk-index here.

We need to implement a Visitor and DistancesUnordered implementation for the flat scan over the PQ compressed vectors in the disk provider.