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 boolean isEmptyDoc(RootObjectMapper root, XContentParser parser) throws IOException {
        if (root.isEnabled()) {
            final XContentParser.Token token = parser.nextToken();
            switch (token) {
                case END_OBJECT:
                    
                    return true;
                case FIELD_NAME:
                    return false;
                default:
                    throwOnMalformedContent(parser);
            }
        }
        return false;
    }
    private static void throwOnMalformedContent(XContentParser parser) {
        throw new DocumentParsingException(
            parser.getTokenLocation(),
            "Malformed content, after first object, either the type field or the actual properties should exist"
        );
    }
    private static DocumentParsingException wrapInDocumentParsingException(DocumentParserContext context, Exception e) {
        
        if (e instanceof DocumentParsingException) {
            return (DocumentParsingException) e;
        }
        return new DocumentParsingException(context.parser().getTokenLocation(), "failed to parse: " + e.getMessage(), e);
    }
    static Mapping createDynamicUpdate(DocumentParserContext context) {
        if (context.getDynamicMappers().isEmpty() && context.getDynamicRuntimeFields().isEmpty()) {