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.endObject();
        }));
        Mapping mapping = doc.dynamicMappingsUpdate();
        assertNotNull(mapping);
        Mapper fooMapper = mapping.getRoot().getMapper("foo");
        assertNotNull(fooMapper);
        assertTrue(fooMapper instanceof ObjectMapper);
        assertNotNull(((ObjectMapper) fooMapper).getMapper("field"));
        Mapper barMapper = ((ObjectMapper) fooMapper).getMapper("bar");
        assertTrue(barMapper instanceof ObjectMapper);
        assertNotNull(((ObjectMapper) barMapper).getMapper("baz"));
    }
    public void testEmptyObjectGetsMapped() throws Exception {
        MapperService mapperService = createMapperService();
        DocumentMapper docMapper = mapperService.documentMapper();
        ParsedDocument doc = docMapper.parse(source(b -> {
            b.startObject("foo");
            b.endObject();
        }));
        Mapping mapping = doc.dynamicMappingsUpdate();
        assertNotNull(mapping);
        Mapper foo = mapping.getRoot().getMapper("foo");
        assertThat(foo, instanceOf(ObjectMapper.class));
        assertEquals(0, ((ObjectMapper) foo).mappers.size());
    }
    public void testDynamicGeoPointArrayWithTemplate() throws Exception {
        DocumentMapper mapper = createDocumentMapper(topMapping(b -> {
            b.startArray("dynamic_templates");
            {
                b.startObject();