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
ParsedDocument doc = mapper.parse(source(b -> b.startObject("foo.bar.baz").field("a", 0).endObject()));
        assertEquals(1, doc.rootDoc().getFields("foo.bar.baz.a").size());
        Mapper fooMapper = doc.dynamicMappingsUpdate().getRoot().getMapper("foo");
        assertNotNull(fooMapper);
        assertThat(fooMapper, instanceOf(ObjectMapper.class));
        Mapper barMapper = ((ObjectMapper) fooMapper).getMapper("bar");
        assertNotNull(barMapper);
        assertThat(barMapper, instanceOf(ObjectMapper.class));
        Mapper bazMapper = ((ObjectMapper) barMapper).getMapper("baz");
        assertNotNull(bazMapper);
        assertThat(bazMapper, instanceOf(ObjectMapper.class));
        Mapper aMapper = ((ObjectMapper) bazMapper).getMapper("a");
        assertNotNull(aMapper);
        assertThat(aMapper, instanceOf(NumberFieldMapper.class));
    }
    public void testDynamicDottedFieldNameObjectWithParentTemplate() throws Exception {
        DocumentMapper mapper = createDocumentMapper(topMapping(b -> {
            b.startArray("dynamic_templates");
            {
                b.startObject();
                {
                    b.startObject("georule");
                    {
                        b.field("match", "foo*");
                        b.startObject("mapping").field("type", "object").endObject();
                    }
                    b.endObject();
                }
                b.endObject();
            }