Fix "size" field in the body of AbstractBulkByScrollRequest

[PnPie]

The "size" field in the body of AbstractBulkByScrollRequest(DeleteByQuery/UpdateByQuery) is not taken into consideration anymore during some previous changes, and currently only passing "size" as parameter works for DeleteByQuery or UpdateByQuery. This is to fix it and make it worked again.

Close #35636

[elasticmachine]

Pinging @elastic/es-distributed

[nik9000]

I think this is almost right, but it has an issue for transport client users. I think. The trouble is that we didn't have any real testing of the default transport client behavior. But I'm fairly sure we should keep that default size setting. And that means changing Abstract rest controlled slightly. But I think this is on the right track!

[nik9000]

I think this should stay as it was or else transport client users will start to get a default scroll size of 10 again which won't perform well..

[PnPie]

Hi @nik9000 I changed search XContent body parsing part and passed a setSize consumer inside to be able to "consume" size field in search request's body.
Otherwise it seems a little bit tricky on Abstract rest level to idenfity whether the size field has been set or not (like this), as by default it's 1000.
What do you think of this ?

[henningandersen]

@PnPie thanks for your efforts on providing this solution. Unfortunately, we have since implemented a different solution in #41894 where we renamed size to max_docs, fixing confusion on the different size parameters in place. I will therefore close this PR.