refactor(search_family): Add Aggregator class

[BagritsevichStepan]

related to this comment

1. Add Aggregator class.
2. Remove redundant std::string copies entirely during the aggregation steps.

TODO:

1. Add logs
2. Remove the usage of std::function

[BorysTheDev]

why do we return true if elements are equal for less operator? As I understand you could take previous implementation and return something like "res != descending"

[BagritsevichStepan]

Yes, it makes sense to return descending here. But just to note, we are not guaranteeing any strict ordering in the result when elements are equal. This is because the final order also depends on the initial distribution of keys across shards (results from shards are joined into one vector, and then a sort is performed on this array).

So, it is not a "stable" sort, meaning that the initial order of the data is influenced by the key distribution across shards, which may give the user the impression of an unstable sort

Upd.: After discussion, it was decided to return false if the elements are equal.

[BagritsevichStepan]

I simplified the comparator logic, so I don't think it’s necessary to use std::less here as we discussed.