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
}
        if (nested.isIncludeInRoot()) {
            LuceneDocument rootDoc = context.rootDoc();
            
            if (nested.isIncludeInParent() == false || parentDoc != rootDoc) {
                addFields(indexVersion, nestedDoc, rootDoc);
            }
        }
    }
    private static void addFields(IndexVersion indexCreatedVersion, LuceneDocument nestedDoc, LuceneDocument rootDoc) {
        String nestedPathFieldName = NestedPathFieldMapper.name(indexCreatedVersion);
        for (IndexableField field : nestedDoc.getFields()) {
            if (field.name().equals(nestedPathFieldName) == false && field.name().equals(IdFieldMapper.NAME) == false) {
                rootDoc.add(field);
            }
        }
    }
    static void parseObjectOrField(DocumentParserContext context, Mapper mapper) throws IOException {
        if (mapper instanceof ObjectMapper objectMapper) {
            context = context.createChildContext(objectMapper);
            parseObjectOrNested(context);
        } else if (mapper instanceof FieldMapper fieldMapper) {
            fieldMapper.parse(context);
            if (context.isWithinCopyTo() == false) {
                List<String> copyToFields = fieldMapper.copyTo().copyToFields();
                if (copyToFields.isEmpty() == false) {
                    XContentParser.Token currentToken = context.parser().currentToken();
                    if (currentToken.isValue() == false && currentToken != XContentParser.Token.VALUE_NULL) {
                        
                        throwOnCopyToOnObject(mapper, copyToFields, context);