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
@Deprecated
    public final DocumentParserContext switchParser(XContentParser parser) {
        return new Wrapper(this.parent, this) {
            @Override
            public XContentParser parser() {
                return parser;
            }
        };
    }
    /**
     * The collection of dimensions for this document.
     */
    public DocumentDimensions getDimensions() {
        return dimensions;
    }
    public abstract ContentPath path();
    /**
     * Creates a context to build dynamic mappers
     */
    public final MapperBuilderContext createDynamicMapperBuilderContext() {
        String p = path().pathAsText("");
        if (p.endsWith(".")) {
            p = p.substring(0, p.length() - 1);
        }
        return new MapperBuilderContext(p, mappingLookup().isSourceSynthetic());
    }
    public abstract XContentParser parser();
    public abstract LuceneDocument rootDoc();
    public abstract LuceneDocument doc();
    protected abstract void addDoc(LuceneDocument doc);