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.field("test2", "value2");
                b.startObject("inner").field("inner_field", "inner_value").endObject();
            }
            b.endObject();
        }));
        assertThat(doc.rootDoc().get("type.type"), equalTo("type_value"));
        assertThat(doc.rootDoc().get("type.test1"), equalTo("value1"));
        assertThat(doc.rootDoc().get("type.test2"), equalTo("value2"));
        assertThat(doc.rootDoc().get("type.inner.inner_field"), equalTo("inner_value"));
    }
    public void testNoLevelWithFieldTypeAsObjectNotFirst() throws Exception {
        DocumentMapper defaultMapper = createDocumentMapper(mapping(b -> {}));
        ParsedDocument doc = defaultMapper.parse(source(b -> {
            b.field("test1", "value1");
            b.startObject("type").field("type_field", "type_value").endObject();
            b.field("test2", "value2");
            b.startObject("inner").field("inner_field", "inner_value").endObject();
        }));
        
        assertThat(doc.rootDoc().get("type.type_field"), equalTo("type_value"));
        assertThat(doc.rootDoc().get("test1"), equalTo("value1"));
        assertThat(doc.rootDoc().get("test2"), equalTo("value2"));
        assertThat(doc.rootDoc().get("inner.inner_field"), equalTo("inner_value"));
    }
    public void testTypeLevelWithFieldTypeAsObjectNotFirst() throws Exception {
        DocumentMapper defaultMapper = createDocumentMapper(mapping(b -> {}));
        ParsedDocument doc = defaultMapper.parse(source(b -> {
            b.startObject("type");
            {
                b.field("test1", "value1");