fix(types): add abTestID to SearchResponse

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Summary

Ticket: https://algolia.atlassian.net/browse/CR-1581

Fixes #1418

Add abTestID field to the SearchResponse type.

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Looking at https://www.algolia.com/doc/api-reference/api-parameters/getRankingInfo/#impact-on-the-response it's a bit confusing if it's under _rankingInfo or at the root, we've put it at the root in v5 which makes sense as it's bound to abTestVariantID, wdyt?

[bodinsamuel]

Testing gives me _rankingInfo in the hits and other fields at the root. The doc is indeed not perfectly clear.

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Yep indeed, I'll add a ticket in our maintenance backlog