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 (token == null) {
                throwEOF(context.parent(), context);
            }
            switch (token) {
                case FIELD_NAME:
                    currentFieldName = parser.currentName();
                    if (currentFieldName.isEmpty()) {
                        throw new IllegalArgumentException("Field name cannot be an empty string");
                    }
                    if (currentFieldName.isBlank()) {
                        throwFieldNameBlank(context, currentFieldName);
                    }
                    break;
                case START_OBJECT:
                    parseObject(context, currentFieldName);
                    break;
                case START_ARRAY:
                    parseArray(context, currentFieldName);
                    break;
                case VALUE_NULL:
                    parseNullValue(context, currentFieldName);
                    break;
                default:
                    if (token.isValue()) {
                        parseValue(context, currentFieldName);
                    }
                    break;
            }
            token = parser.nextToken();
        }