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
public void testTypeless() throws IOException {
        String mapping = Strings.toString(
            XContentFactory.jsonBuilder()
                .startObject()
                .startObject("type")
                .startObject("properties")
                .startObject("foo")
                .field("type", "keyword")
                .endObject()
                .endObject()
                .endObject()
                .endObject()
        );
        DocumentMapper mapper = createDocumentMapper(mapping);
        ParsedDocument doc = mapper.parse(source(b -> b.field("foo", "1234")));
        assertNull(doc.dynamicMappingsUpdate()); 
    }
    public void testParseWithFlattenedField() throws IOException {
        DocumentMapper mapper = createDocumentMapper(fieldMapping(b -> b.field("type", "flattened")));
        ParsedDocument doc = mapper.parse(source(b -> {
            b.startObject("field");
            b.field("first", "first");
            b.field("second", "second");
            b.endObject();
        }));
        assertNull(doc.dynamicMappingsUpdate());
        assertNotNull(doc.rootDoc().getField("field"));
        assertNull(doc.rootDoc().getField("field.first"));
        assertNull(doc.rootDoc().getField("field.second"));
    }