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
) throws IOException {
            super(
                mappingLookup,
                mappingParserContext,
                source,
                mappingLookup.getMapping().getRoot(),
                ObjectMapper.Dynamic.getRootDynamic(mappingLookup)
            );
            if (mappingLookup.getMapping().getRoot().subobjects()) {
                this.parser = DotExpandingXContentParser.expandDots(parser, this.path);
            } else {
                this.parser = parser;
            }
            this.document = new LuceneDocument();
            this.documents.add(document);
            this.maxAllowedNumNestedDocs = indexSettings().getMappingNestedDocsLimit();
            this.numNestedDocs = 0L;
        }
        @Override
        public Mapper getMapper(String name) {
            Mapper mapper = getMetadataMapper(name);
            if (mapper != null) {
                return mapper;
            }
            return super.getMapper(name);
        }
        @Override
        public ContentPath path() {
            return this.path;
        }