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
}
                    parseCopyFields(context, copyToFields);
                }
            }
        } else if (mapper instanceof FieldAliasMapper) {
            throwOnCopyToOnFieldAlias(context, mapper);
        } else {
            throwOnUnrecognizedMapperType(mapper);
        }
    }
    private static void throwOnUnrecognizedMapperType(Mapper mapper) {
        throw new IllegalStateException(
            "The provided mapper [" + mapper.name() + "] has an unrecognized type [" + mapper.getClass().getSimpleName() + "]."
        );
    }
    private static void throwOnCopyToOnFieldAlias(DocumentParserContext context, Mapper mapper) {
        throw new DocumentParsingException(
            context.parser().getTokenLocation(),
            "Cannot " + (context.isWithinCopyTo() ? "copy" : "write") + " to a field alias [" + mapper.name() + "]."
        );
    }
    private static void throwOnCopyToOnObject(Mapper mapper, List<String> copyToFields, DocumentParserContext context) {
        throw new DocumentParsingException(
            context.parser().getTokenLocation(),
            "Cannot copy field ["
                + mapper.name()
                + "] to fields "
                + copyToFields
                + ". Copy-to currently only works for value-type fields, not objects."
        );