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/DocumentParserContext.java
/**
     * Returns an Iterable over all non-root documents. If there are no non-root documents
     * the iterable will return an empty iterator.
     */
    public abstract Iterable<LuceneDocument> nonRootDocuments();
    /**
     * @return a RootObjectMapper.Builder to be used to construct a dynamic mapping update
     */
    public final RootObjectMapper.Builder updateRoot() {
        return mappingLookup.getMapping().getRoot().newBuilder(mappingParserContext.getIndexSettings().getIndexVersionCreated());
    }
    public boolean isWithinCopyTo() {
        return false;
    }
    public final DocumentParserContext createChildContext(ObjectMapper parent) {
        return new Wrapper(parent, this);
    }
    /**
     * Return a new context that will be used within a nested document.
     */
    public final DocumentParserContext createNestedContext(NestedObjectMapper nestedMapper) {
        if (isWithinCopyTo()) {
            
            return this;
        }
        final LuceneDocument doc = new LuceneDocument(nestedMapper.fullPath(), doc());