Currently, when an object is mapped with `subobjects: false`, trying to ingest a document that has nested objects leads to a mapping exception.

In a couple of occasions, we've been talking about that it would be convenient if Elasticsearch could automatically flatten the fields instead of the client having to know about the `subobjects` setting. This would make `subobjects: false` more of a drop-in replacement that allows for a much easier adoption, without changes in the client.

Note that the `_source` should ideally stay untouched, just the stored fields should be flattened automatically.

Some related issues
* https://github.com/elastic/apm/issues/347
* https://github.com/elastic/elasticsearch/issues/88777
* https://github.com/elastic/package-spec/issues/349
server/src/main/java/org/elasticsearch/index/mapper/DocumentParser.java
if (docsReversed) {
                throw new IllegalStateException("documents are already reversed");
            }
            return documents.subList(1, documents.size());
        }
        /**
         * Returns a copy of the provided {@link List} where parent documents appear
         * after their children.
         */
        private List<LuceneDocument> reorderParentAndGetDocs() {
            if (documents.size() > 1 && docsReversed == false) {
                docsReversed = true;
                
                List<LuceneDocument> newDocs = new ArrayList<>(documents.size());
                LinkedList<LuceneDocument> parents = new LinkedList<>();
                for (LuceneDocument doc : documents) {
                    while (parents.peek() != doc.getParent()) {
                        newDocs.add(parents.poll());
                    }
                    parents.add(0, doc);
                }
                newDocs.addAll(parents);
                documents.clear();
                documents.addAll(newDocs);
            }
            return documents;
        }
    }
}