Approximate nearest neighbours algorithms

[BERENZ]

List of backends for approximate nearest neighbours (ANN) search:

• RcppHNSW::RcppHNSW
• RcppAnnoy
• mlpack with lsh, knn, krann
• RANN
• rnndescent

[BERENZ]

RANN will not be supported as mlpack has knn with kdtrees.

[BERENZ]

rnndescen moved to later

[BERENZ]

Add support for k_search with default 30 for each algorithm.