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
}));
        assertEquals(2, doc.rootDoc().getFields("field").size());
    }
    public void testMappedLongArray() throws Exception {
        DocumentMapper mapper = createDocumentMapper(fieldMapping(b -> b.field("type", "long")));
        ParsedDocument doc = mapper.parse(source(b -> b.startArray("field").value(0).value(1).endArray()));
        assertEquals(2, doc.rootDoc().getFields("field").size());
    }
    public void testDynamicObjectWithTemplate() throws Exception {
        DocumentMapper mapper = createDocumentMapper(topMapping(b -> {
            b.startArray("dynamic_templates");
            {
                b.startObject();
                {
                    b.startObject("georule");
                    {
                        b.field("match", "foo*");
                        b.startObject("mapping");
                        {
                            b.field("type", "object");
                            b.startObject("properties");
                            {
                                b.startObject("bar").field("type", "keyword").endObject();
                            }
                            b.endObject();
                        }
                        b.endObject();
                    }
                    b.endObject();
                }