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/DocumentParserContext.java
*/
    public final DocumentParserContext createCopyToContext(String copyToField, LuceneDocument doc) throws IOException {
        ContentPath path = new ContentPath();
        XContentParser parser = DotExpandingXContentParser.expandDots(new CopyToParser(copyToField, parser()), path);
        return new Wrapper(root(), this) {
            @Override
            public ContentPath path() {
                return path;
            }
            @Override
            public XContentParser parser() {
                return parser;
            }
            @Override
            public boolean isWithinCopyTo() {
                return true;
            }
            @Override
            public LuceneDocument doc() {
                return doc;
            }
        };
    }
    /**
     *  @deprecated we are actively deprecating and removing the ability to pass
     *              complex objects to multifields, so try and avoid using this method
     * Replace the XContentParser used by this context
     * @param parser    the replacement parser
     * @return  a new context with a replaced parser
     */