### 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
DocumentMapper documentMapper = createDocumentMapper(topMapping(b -> {
            b.field("dynamic", false);
            b.startObject("properties");
            b.startObject("dynamic_true").field("type", "object").field("dynamic", true).endObject();
            b.startObject("dynamic_runtime").field("type", "object").field("dynamic", ObjectMapper.Dynamic.RUNTIME).endObject();
            b.endObject();
        }));
        ParsedDocument doc = documentMapper.parse(source(b -> {
            b.startObject("anything").field("field", "text").endObject();
            b.startObject("dynamic_true").field("field1", "text").startObject("child").field("field2", "text").endObject().endObject();
            b.startObject("dynamic_runtime").field("field3", "text").startObject("child").field("field4", "text").endObject().endObject();
        }));
        assertEquals(XContentHelper.stripWhitespace("""
            {
              "_doc": {
                "dynamic": "false",
                "runtime": {
                  "dynamic_runtime.child.field4": {
                    "type": "keyword"
                  },
                  "dynamic_runtime.field3": {
                    "type": "keyword"
                  }
                },
                "properties": {
                  "dynamic_true": {
                    "dynamic": "true",
                    "properties": {
                      "child": {
                        "properties": {