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
return null;
        }
        RootObjectMapper.Builder rootBuilder = context.updateRoot();
        for (Mapper mapper : context.getDynamicMappers()) {
            rootBuilder.addDynamic(mapper.name(), null, mapper, context);
        }
        for (RuntimeField runtimeField : context.getDynamicRuntimeFields()) {
            rootBuilder.addRuntimeField(runtimeField);
        }
        RootObjectMapper root = rootBuilder.build(MapperBuilderContext.root(context.mappingLookup().isSourceSynthetic()));
        return context.mappingLookup().getMapping().mappingUpdate(root);
    }
    static void parseObjectOrNested(DocumentParserContext context) throws IOException {
        if (context.parent().isEnabled() == false) {
            context.parser().skipChildren();
            return;
        }
        XContentParser parser = context.parser();
        XContentParser.Token token = parser.currentToken();
        if (token == XContentParser.Token.VALUE_NULL) {
            
            return;
        }
        String currentFieldName = parser.currentName();
        if (token.isValue()) {
            throwOnConcreteValue(context.parent(), currentFieldName, context);
        }
        if (context.parent().isNested()) {
            context = context.createNestedContext((NestedObjectMapper) context.parent());
        }