[FEA] Support int64_t nearest neighbor index type for all ANN algorithm.

[tfeher]

Currently most of our the nearest neighbors methods support "int64_t" type for the nearest neighbor indices. The only exception is CAGRA, that returns uint32_t indices. For consistency, it would be great to support int64_t for CAGRA as well, and it can be done with a small overhead by adding a simple mapping after the search.

Similarly, it would be easy to add uint32_t support for all the other nearest neighbor algorithms, if there is a need for that.