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
IndexableField idField = doc.getParent().getField(IdFieldMapper.NAME);
        if (idField != null) {
            
            
            
            doc.add(new StringField(IdFieldMapper.NAME, idField.binaryValue(), Field.Store.NO));
        } else {
            throw new IllegalStateException("The root document of a nested document should have an _id field");
        }
        doc.add(NestedPathFieldMapper.field(indexSettings().getIndexVersionCreated(), nestedMapper.nestedTypePath()));
        addDoc(doc);
        return switchDoc(doc);
    }
    /**
     * Return a new context that has the provided document as the current document.
     */
    public final DocumentParserContext switchDoc(final LuceneDocument document) {
        return new Wrapper(this.parent, this) {
            @Override
            public LuceneDocument doc() {
                return document;
            }
        };
    }
    /**
     * Return a context for copy_to directives
     * @param copyToField   the name of the field to copy to
     * @param doc           the document to target