### 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/test/java/org/elasticsearch/index/mapper/DynamicMappingTests.java
"object": {
                    "properties": {
                      "foo": {
                        "properties": {
                          "bar": {
                            "properties": {
                              "baz": {
                                "type": "text",
                                "fields": {
                                  "keyword": {
                                    "type": "keyword",
                                    "ignore_above": 256
                                  }
                                }
                              }
                            }
                          }
                        }
                      }
                    }
                  }
                }
              }
            }"""), Strings.toString(doc.dynamicMappingsUpdate()));
    }
    public void testArray() throws Exception {
        DocumentMapper mapper = createDocumentMapper(mapping(b -> {}));
        ParsedDocument doc = mapper.parse(source(b -> b.startArray("foo").value("bar").value("baz").endArray()));
        assertNotNull(doc.dynamicMappingsUpdate());
        assertThat(Strings.toString(doc.dynamicMappingsUpdate()), containsString("""