Improve SimpleMongoRepository.saveAll to use batch updates

[syjdev]

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Closes #5220

[mp911de]

As per #5220 (comment), we should seek a consistent bulk approach for inserts and updates, also considering the reactive implementation.

[syjdev]

Thank you for the feedback. I'll revise the implementation based on both your comment on #5220 and the feedback you've provided here. Thanks!

[syjdev]

I moved both the imperative and the reactive saveAll(Iterable) onto
bulkWrite(Bulk, BulkWriteOptions).

Also, I changed BulkWriter/ReactiveBulkWriter to propagate the generated _id
back to the entity after the write because, unlike doInsert(…), the bulk insert
path did not.

[syjdev]

saveAll(…) now returns the instances produced by the write, so immutable entities come back with their generated _id.

This requires a new BulkWriteResult.savedEntities(), which throws UnsupportedOperationException for results that do not track entities. Let me know if you would prefer a different API shape.