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
dynamicObjectMapper = new NoOpObjectMapper(currentFieldName, context.path().pathAsText(currentFieldName));
            } else {
                dynamicObjectMapper = DynamicFieldsBuilder.createDynamicObjectMapper(context, currentFieldName);
                context.addDynamicMapper(dynamicObjectMapper);
            }
            if (context.parent().subobjects() == false) {
                if (dynamicObjectMapper instanceof NestedObjectMapper) {
                    throw new DocumentParsingException(
                        context.parser().getTokenLocation(),
                        "Tried to add nested object ["
                            + dynamicObjectMapper.simpleName()
                            + "] to object ["
                            + context.parent().name()
                            + "] which does not support subobjects"
                    );
                }
                if (dynamicObjectMapper instanceof ObjectMapper) {
                    throw new DocumentParsingException(
                        context.parser().getTokenLocation(),
                        "Tried to add subobject ["
                            + dynamicObjectMapper.simpleName()
                            + "] to object ["
                            + context.parent().name()
                            + "] which does not support subobjects"
                    );
                }
            }
            if (dynamicObjectMapper instanceof NestedObjectMapper && context.isWithinCopyTo()) {
                throwOnCreateDynamicNestedViaCopyTo(dynamicObjectMapper, context);
            }