[FEA] Establish streaming incremental micro-batched index build for CAGRA

Oracle has asked that we overlap batches as much as possible to support the ability to stream batches into an API so that we can start building a large CAGRA graph on the batches prior to the last batch being received. The idea is that we immediately start building so that we don't have to wait for the last batch to arrive.

We need to find a good solution for this, because it's going to keep coming up as we push more heavily on serverless and micro-service infrastructures.

One idea is that we once again provide a separate optimize() function in cuvs::neighbors::cagra::helpers so that we can build the all-neighbors graph in a streaming fashion (using a variant of our new cuvs::neighbors::all_neighbors APIs) and then run that graph through the optimization process.

This, of course, assumes the optimization process doesn't end up becoming the bottleneck. If it does, we might need to separate that our further.