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
parseDynamicValue(context, currentFieldName);
        }
    }
    private static void throwOnNoFieldName(DocumentParserContext context) throws IOException {
        throw new DocumentParsingException(
            context.parser().getTokenLocation(),
            "object mapping ["
                + context.parent().name()
                + "] trying to serialize a value with"
                + " no field associated with it, current value ["
                + context.parser().textOrNull()
                + "]"
        );
    }
    private static void parseNullValue(DocumentParserContext context, String lastFieldName) throws IOException {
        
        Mapper mapper = getLeafMapper(context, lastFieldName);
        if (mapper != null) {
            
            parseObjectOrField(context, mapper);
        } else if (context.dynamic() == ObjectMapper.Dynamic.STRICT) {
            throw new StrictDynamicMappingException(context.parser().getTokenLocation(), context.parent().fullPath(), lastFieldName);
        }
    }
    private static void parseDynamicValue(final DocumentParserContext context, String currentFieldName) throws IOException {
        if (context.dynamic() == ObjectMapper.Dynamic.STRICT) {
            throw new StrictDynamicMappingException(context.parser().getTokenLocation(), context.parent().fullPath(), currentFieldName);
        }
        if (context.dynamic() == ObjectMapper.Dynamic.FALSE) {
            failIfMatchesRoutingPath(context, currentFieldName);