### 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/DocumentParser.java
@Override
        public XContentParser parser() {
            return this.parser;
        }
        @Override
        public LuceneDocument rootDoc() {
            return documents.get(0);
        }
        @Override
        public LuceneDocument doc() {
            return this.document;
        }
        @Override
        protected void addDoc(LuceneDocument doc) {
            numNestedDocs++;
            if (numNestedDocs > maxAllowedNumNestedDocs) {
                throw new DocumentParsingException(
                    parser.getTokenLocation(),
                    "The number of nested documents has exceeded the allowed limit of ["
                        + maxAllowedNumNestedDocs
                        + "]."
                        + " This limit can be set by changing the ["
                        + MapperService.INDEX_MAPPING_NESTED_DOCS_LIMIT_SETTING.getKey()
                        + "] index level setting."
                );
            }
            this.documents.add(doc);
        }
        @Override
        public Iterable<LuceneDocument> nonRootDocuments() {