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
}
    private static void parseObject(final DocumentParserContext context, String currentFieldName) throws IOException {
        assert currentFieldName != null;
        Mapper objectMapper = context.getMapper(currentFieldName);
        if (objectMapper != null) {
            context.path().add(currentFieldName);
            if (objectMapper instanceof ObjectMapper objMapper) {
                if (objMapper.subobjects() == false) {
                    context.path().setWithinLeafObject(true);
                }
            }
            parseObjectOrField(context, objectMapper);
            context.path().setWithinLeafObject(false);
            context.path().remove();
        } else {
            parseObjectDynamic(context, currentFieldName);
        }
    }
    private static void parseObjectDynamic(DocumentParserContext context, String currentFieldName) throws IOException {
        if (context.dynamic() == ObjectMapper.Dynamic.STRICT) {
            throw new StrictDynamicMappingException(context.parser().getTokenLocation(), context.parent().fullPath(), currentFieldName);
        } else if (context.dynamic() == ObjectMapper.Dynamic.FALSE) {
            failIfMatchesRoutingPath(context, currentFieldName);
            
            context.parser().skipChildren();
        } else {
            Mapper dynamicObjectMapper;
            if (context.dynamic() == ObjectMapper.Dynamic.RUNTIME) {