Allow combining index sorting and nested mappings.

[jpountz]

Description

Until now, we disallowed configuring nested fields in mappings if an index sort was configured as the index sort could break the way that we organize doc IDs.

Lucene recently fixed this by enabling recording root documents via IndexWriterConfig#setParentField. When a parent field is configured, index sorting will make sure to preserve the block structure. We should now remove the Elasticsearch limitation by removing the check and configuring a parent field when index sorting is enabled. This would then allow nested mappings to be added.

[elasticsearchmachine]

Pinging @elastic/es-search (Team:Search)

[itepifanio]

@jpountz does Elastic have any workaround to bypass this behavior?

I need to access nested information but I'm paginating a whole index and using the sort and search_after to do so.

def scrolling(...):
    result = elastic_client.search(
        index=self.index,
        size=self.batch_size,
        search_after=self._search_after,
        sort=self._sort
    ),

    hits = result.get("hits", {}).get("hits", [])
    if not hits:
        return
   
    self._search_after = hits[-1]["sort"]