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
context.path().add(currentFieldName);
            if (dynamicObjectMapper instanceof ObjectMapper objectMapper) {
                if (objectMapper.subobjects() == false) {
                    context.path().setWithinLeafObject(true);
                }
            }
            parseObjectOrField(context, dynamicObjectMapper);
            context.path().setWithinLeafObject(false);
            context.path().remove();
        }
    }
    private static void throwOnCreateDynamicNestedViaCopyTo(Mapper dynamicObjectMapper, DocumentParserContext context) {
        throw new DocumentParsingException(
            context.parser().getTokenLocation(),
            "It is forbidden to create dynamic nested objects ([" + dynamicObjectMapper.name() + "]) through `copy_to`"
        );
    }
    private static void parseArray(DocumentParserContext context, String lastFieldName) throws IOException {
        Mapper mapper = getLeafMapper(context, lastFieldName);
        if (mapper != null) {
            
            
            
            if (parsesArrayValue(mapper)) {
                parseObjectOrField(context, mapper);
            } else {
                parseNonDynamicArray(context, lastFieldName, lastFieldName);
            }
        } else {
            if (context.dynamic() == ObjectMapper.Dynamic.STRICT) {