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
}
        }));
        
        DocumentLeafReader reader = new DocumentLeafReader(context.rootDoc(), fieldScripts);
        for (Consumer<LeafReaderContext> script : fieldScripts.values()) {
            script.accept(reader.getContext());
        }
    }
    private static void validateStart(XContentParser parser) throws IOException {
        
        XContentParser.Token token = parser.nextToken();
        if (token != XContentParser.Token.START_OBJECT) {
            throwNoStartOnObject();
        }
    }
    private static void throwNoStartOnObject() {
        throw new DocumentParsingException(XContentLocation.UNKNOWN, "Malformed content, must start with an object");
    }
    private static void validateEnd(XContentParser parser) throws IOException {
        XContentParser.Token token;
        
        
        token = parser.nextToken();
        if (token != null) {
            throwNotAtEnd(token);
        }
    }
    private static void throwNotAtEnd(XContentParser.Token token) {
        throw new IllegalArgumentException("Malformed content, found extra data after parsing: " + token);
    }