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.endArray();
        }));
        ParsedDocument doc = mapper.parse(source(b -> b.field("foo.bar", 42)));
        assertEquals(42L, doc.docs().get(0).getNumericValue("foo.bar"));
    }
    public void testNestedHaveIdAndTypeFields() throws Exception {
        DocumentMapper mapper = createDocumentMapper(mapping(b -> {
            b.startObject("foo");
            {
                b.field("type", "nested");
                b.startObject("properties");
                {
                    b.startObject("bar").field("type", "keyword").endObject();
                }
                b.endObject();
            }
            b.endObject();
            b.startObject("baz").field("type", "keyword").endObject();
        }));
        
        ParsedDocument result = mapper.parse(source(b -> {
            b.startArray("foo");
            {
                b.startObject().field("bar", "value1").endObject();
            }
            b.endArray();
            b.field("baz", "value2");
        }));
        assertEquals(2, result.docs().size());