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
/**
     * Wraps a given context while allowing to override some of its behaviour by re-implementing some of the non final methods
     */
    private static class Wrapper extends DocumentParserContext {
        private final DocumentParserContext in;
        private Wrapper(ObjectMapper parent, DocumentParserContext in) {
            super(parent, parent.dynamic == null ? in.dynamic : parent.dynamic, in);
            this.in = in;
        }
        @Override
        public Iterable<LuceneDocument> nonRootDocuments() {
            return in.nonRootDocuments();
        }