Fix slices on reindex APIs

[pquentin]

There are four reindex APIs, and this fixes 3/4 by adding slices to the response.

Server code: https://github.com/elastic/elasticsearch/blob/cdebb13d2cd2f506d316581a3cb3ef98dd55db96/server/src/main/java/org/elasticsearch/index/reindex/BulkByScrollTask.java#L537-L576. Not backporting as we're moving a class from a namespace to another.

I did not fix reindex_rethrottle all the way because it's more complicated!

[github-actions]

Following you can find the validation changes against the target branch for the APIs.

API	Status	Request	Response
delete_by_query	🔴 → 🟢	27/27	24/26 → 26/26
reindex	🔴 → 🟢	79/79	76/78 → 78/78
reindex_rethrottle	🔴	10/10	0/10 → 6/10
update_by_query	🔴 → 🟢	42/42	39/41 → 41/41

You can validate these APIs yourself by using the make validate target.