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
@Override
        public boolean isWithinCopyTo() {
            return in.isWithinCopyTo();
        }
        @Override
        public ContentPath path() {
            return in.path();
        }
        @Override
        public XContentParser parser() {
            return in.parser();
        }
        @Override
        public LuceneDocument rootDoc() {
            return in.rootDoc();
        }
        @Override
        public LuceneDocument doc() {
            return in.doc();
        }
        @Override
        protected void addDoc(LuceneDocument doc) {
            in.addDoc(doc);
        }
    }
    private final MappingLookup mappingLookup;
    private final MappingParserContext mappingParserContext;
    private final SourceToParse sourceToParse;
    private final Set<String> ignoredFields;
    private final List<Mapper> dynamicMappers;