### 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
assertThat(doc.rootDoc().get("field1"), equalTo("value1"));
        assertThat(doc.rootDoc().get("field2"), equalTo("value2"));
        assertEquals(XContentHelper.stripWhitespace("""
            {
              "_doc": {
                "dynamic": "true",
                "properties": {
                  "field2": {
                    "type": "text",
                    "fields": {
                      "keyword": {
                        "type": "keyword",
                        "ignore_above": 256
                      }
                    }
                  }
                }
              }
            }"""), Strings.toString(doc.dynamicMappingsUpdate()));
    }
    public void testDynamicRuntime() throws IOException {
        DocumentMapper defaultMapper = createDocumentMapper(
            dynamicMapping("runtime", b -> b.startObject("field1").field("type", "text").endObject())
        );
        ParsedDocument doc = defaultMapper.parse(source(b -> {
            b.field("field1", "value1");
            b.field("field2", "value2");
        }));
        assertThat(doc.rootDoc().get("field1"), equalTo("value1"));
        assertNull(doc.rootDoc().get("field2"));