Allow to index documents to different indices in a bulk request

[sothawo]

When doing a bulk request, all index or update queries are sent to the index specified by the entity or as parameter of the corresponding bulk... method from DocumentOperations.

It should be possible to specify different index names for the single operations in a bulk request (This was possible back in version 3.x).

SO question bringing this up: https://stackoverflow.com/questions/70510095/spring-data-elasticsearch-4-bulk-insert-different-queries-to-different-indices