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
libs/x-content/src/main/java/org/elasticsearch/xcontent/XContentSubParser.java
public void skipChildren() throws IOException {
        Token token = currentToken();
        if (token != Token.START_OBJECT && token != Token.START_ARRAY) {
            
            return;
        }
        int backToLevel = level - 1;
        while (nextToken() != null) {
            if (level <= backToLevel) {
                return;
            }
        }
    }
    @Override
    public boolean isClosed() {
        return closed;
    }
    @Override
    public void close() throws IOException {
        if (closed == false) {
            closed = true;
            while (true) {
                if (nextToken() == null) {
                    return;
                }
            }
        }
    }
}