Currently, when an object is mapped with `subobjects: false`, trying to ingest a document that has nested objects leads to a mapping exception.

In a couple of occasions, we've been talking about that it would be convenient if Elasticsearch could automatically flatten the fields instead of the client having to know about the `subobjects` setting. This would make `subobjects: false` more of a drop-in replacement that allows for a much easier adoption, without changes in the client.

Note that the `_source` should ideally stay untouched, just the stored fields should be flattened automatically.

Some related issues
* https://github.com/elastic/apm/issues/347
* https://github.com/elastic/elasticsearch/issues/88777
* https://github.com/elastic/package-spec/issues/349
server/src/test/java/org/elasticsearch/index/mapper/DocumentParserTests.java
b.startObject("type").field("type_field", "type_value").endObject();
                b.field("test2", "value2");
                b.startObject("inner").field("inner_field", "inner_value").endObject();
            }
            b.endObject();
        }));
        assertThat(doc.rootDoc().get("type.type.type_field"), equalTo("type_value"));
        assertThat(doc.rootDoc().get("type.test1"), equalTo("value1"));
        assertThat(doc.rootDoc().get("type.test2"), equalTo("value2"));
        assertThat(doc.rootDoc().get("type.inner.inner_field"), equalTo("inner_value"));
    }
    public void testDynamicDateDetectionDisabledOnNumbers() throws IOException {
        DocumentMapper mapper = createDocumentMapper(topMapping(b -> b.startArray("dynamic_date_formats").value("yyyy").endArray()));
        
        
        ParsedDocument doc = mapper.parse(source(b -> b.field("foo", "2016")));
        Mapping update = doc.dynamicMappingsUpdate();
        assertNotNull(update);
        Mapper dateMapper = update.getRoot().getMapper("foo");
        assertNotNull(dateMapper);
        assertThat(dateMapper, not(instanceOf(DateFieldMapper.class)));
    }
    public void testDynamicDateDetectionEnabledWithNoSpecialCharacters() throws IOException {
        DocumentMapper mapper = createDocumentMapper(topMapping(b -> b.startArray("dynamic_date_formats").value("yyyy MM").endArray()));
        
        ParsedDocument doc = mapper.parse(source(b -> b.field("foo", "2016 12")));
        Mapping update = doc.dynamicMappingsUpdate();
        assertNotNull(update);
        Mapper dateMapper = update.getRoot().getMapper("foo");
        assertNotNull(dateMapper);