MB-62182: Fast merge of vector indexes

[Thejas-bhat]

• Introduces a way to merge the vector indexes without having to re-train the merged index.
• Uses the merge_from API from faiss which merges the underlying inverted lists (based on the identical centroid layout) in a block wise fashion.
• During the merge process, if the config tells us to train the vector index (during the initial part of index build before the data ingest) the vectors from the indexes are reconstructed + trained on the merged index. If the config specifies us to use the "centroid index" to merge, we will now use that template for the merged index and also use the merge_from API to move all the vectors from the child indexes to the merged one.