MB-62182: fast merge of vector indexes

[Thejas-bhat]

• The existing merge_from API can be utilized to merge 2 IVF indexes provided they have identical centroid layout.
• To avoid re-training in a segment architecture, a template index can be first created out of a random sample from the data when its size is known and fixed.
• This template can be copied between segments and the inverted lists can be merged without affecting the learned context of the data distribution. This would also involve the merging of the direct map as well which stores the exact location of a key (vector) within the inverted lists.

Comments addressed, re-review.