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 == XContentParser.Token.END_OBJECT) {
            token = parser.nextToken();
        }
        if (token == XContentParser.Token.START_OBJECT) {
            
            parser.nextToken();
        }
        innerParseObject(context);
        
        if (context.parent().isNested()) {
            copyNestedFields(context, (NestedObjectMapper) context.parent());
        }
    }
    private static void throwOnConcreteValue(ObjectMapper mapper, String currentFieldName, DocumentParserContext context) {
        throw new DocumentParsingException(
            context.parser().getTokenLocation(),
            "object mapping for ["
                + mapper.name()
                + "] tried to parse field ["
                + currentFieldName
                + "] as object, but found a concrete value"
        );
    }
    private static void innerParseObject(DocumentParserContext context) throws IOException {
        final XContentParser parser = context.parser();
        XContentParser.Token token = parser.currentToken();
        String currentFieldName = null;
        assert token == XContentParser.Token.FIELD_NAME || token == XContentParser.Token.END_OBJECT;
        while (token != XContentParser.Token.END_OBJECT) {