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("first");
                    {
                        b.field("type", "text");
                        b.field("store", "true");
                        b.field("index", false);
                    }
                    b.endObject();
                }
                b.endObject();
            }
            b.endObject();
        }));
        LuceneDocument doc = docMapper.parse(source(b -> {
            b.startObject("name");
            {
                b.field("first", "shay");
                b.field("last", "banon");
            }
            b.endObject();
        })).rootDoc();
        assertThat(doc.get(docMapper.mappers().getMapper("name.first").name()), equalTo("shay"));
    }
    public void testParseToJsonAndParse() throws Exception {
        String mapping = copyToStringFromClasspath("/org/elasticsearch/index/mapper/simple/test-mapping.json");
        MapperService mapperService = createMapperService(mapping(b -> {}));
        merge(mapperService, mapping);
        String builtMapping = mapperService.documentMapper().mappingSource().string();
        
        DocumentMapper builtDocMapper = createDocumentMapper(builtMapping);
        BytesReference json = new BytesArray(copyToBytesFromClasspath("/org/elasticsearch/index/mapper/simple/test1.json"));