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
LuceneDocument doc = builtDocMapper.parse(new SourceToParse("1", json, XContentType.JSON)).rootDoc();
        assertThat(doc.getBinaryValue(IdFieldMapper.NAME), equalTo(Uid.encodeId("1")));
        assertThat(doc.get(builtDocMapper.mappers().getMapper("name.first").name()), equalTo("shay"));
    }
    public void testSimpleParser() throws Exception {
        String mapping = copyToStringFromClasspath("/org/elasticsearch/index/mapper/simple/test-mapping.json");
        DocumentMapper docMapper = createDocumentMapper(mapping);
        assertThat((String) docMapper.mapping().getMeta().get("param1"), equalTo("value1"));
        BytesReference json = new BytesArray(copyToBytesFromClasspath("/org/elasticsearch/index/mapper/simple/test1.json"));
        LuceneDocument doc = docMapper.parse(new SourceToParse("1", json, XContentType.JSON)).rootDoc();
        assertThat(doc.getBinaryValue(IdFieldMapper.NAME), equalTo(Uid.encodeId("1")));
        assertThat(doc.get(docMapper.mappers().getMapper("name.first").name()), equalTo("shay"));
    }
    public void testSimpleParserNoTypeNoId() throws Exception {
        String mapping = copyToStringFromClasspath("/org/elasticsearch/index/mapper/simple/test-mapping.json");
        DocumentMapper docMapper = createDocumentMapper(mapping);
        BytesReference json = new BytesArray(copyToBytesFromClasspath("/org/elasticsearch/index/mapper/simple/test1-notype-noid.json"));
        LuceneDocument doc = docMapper.parse(new SourceToParse("1", json, XContentType.JSON)).rootDoc();
        assertThat(doc.getBinaryValue(IdFieldMapper.NAME), equalTo(Uid.encodeId("1")));
        assertThat(doc.get(docMapper.mappers().getMapper("name.first").name()), equalTo("shay"));
    }
    public void testAttributes() throws Exception {
        String mapping = copyToStringFromClasspath("/org/elasticsearch/index/mapper/simple/test-mapping.json");
        DocumentMapper docMapper = createDocumentMapper(mapping);
        assertThat((String) docMapper.mapping().getMeta().get("param1"), equalTo("value1"));
        String builtMapping = docMapper.mappingSource().string();
        DocumentMapper builtDocMapper = createDocumentMapper(builtMapping);
        assertThat((String) builtDocMapper.mapping().getMeta().get("param1"), equalTo("value1"));
    }
    public void testNoDocumentSent() throws Exception {