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
Mapper subxMapper = ((ObjectMapper) xMapper).getMapper("subx");
        assertTrue(subxMapper instanceof ObjectMapper);
        assertNotNull(((ObjectMapper) subxMapper).getMapper("foo"));
        assertNull(((ObjectMapper) subxMapper).getMapper("subsubx"));
    }
    public void testDeepSubfieldAfterSubfieldMappingUpdate() throws Exception {
        DocumentMapper docMapper = createDummyMapping();
        ParsedDocument doc = docMapper.parse(source(b -> {
            b.field("x.a", 10);
            b.field("x.subx.b", 10);
        }));
        Mapping mapping = doc.dynamicMappingsUpdate();
        assertNotNull(mapping);
        Mapper xMapper = mapping.getRoot().getMapper("x");
        assertNotNull(xMapper);
        assertTrue(xMapper instanceof ObjectMapper);
        assertNotNull(((ObjectMapper) xMapper).getMapper("a"));
        Mapper subxMapper = ((ObjectMapper) xMapper).getMapper("subx");
        assertTrue(subxMapper instanceof ObjectMapper);
        assertNotNull(((ObjectMapper) subxMapper).getMapper("b"));
    }
    public void testObjectMappingUpdate() throws Exception {
        MapperService mapperService = createMapperService();
        DocumentMapper docMapper = mapperService.documentMapper();
        ParsedDocument doc = docMapper.parse(source(b -> {
            b.startObject("foo");
            b.startObject("bar");
            b.field("baz", 10);
            b.endObject();
            b.field("field", 10);