[FEA] Update MST Reduction Op

[tarang-jain]

Make HDBSCAN MST reduction operation compatible with #1445 raft PR. This PR updates the Mutual Reachability reduction op in HDBSCAN in the following ways:

1. Colors information is no longer needed in the reduction op because the new mst implementation in raft ensures that distances between points of the same color are not computed.
2. Adds gather and scatter functions to rearrange the core distances within the reduction op so that they are aligned with the sort-plan wherein rows in the input matrix and core distances are rearranged so that the training data points are sorted by color.
   Closes [FEA] Make FixConnectivitiesRedOp Compatible with Masked NN for building MST #5456

[cjnolet]

LGTM, pending revert of get_raft.cmake

[cjnolet]

/merge