### Description

Dynamic mappings can greatly help the getting started experience but they currently don't apply to all types, one example is `dense_vector`.

It's been suggested to automatically map float arrays as dense vectors when there are more than 100 values, or a heuristic along these lines.
server/src/main/java/org/elasticsearch/index/mapper/DocumentParserContext.java
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
     */
    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;
            }