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
assertNotNull(result.docs().get(0).getField(IdFieldMapper.NAME));
        assertEquals(Uid.encodeId("1"), result.docs().get(0).getField(IdFieldMapper.NAME).binaryValue());
        assertEquals(StringField.TYPE_NOT_STORED, result.docs().get(0).getField(IdFieldMapper.NAME).fieldType());
        assertNotNull(result.docs().get(0).getField(NestedPathFieldMapper.NAME));
        assertEquals("foo", result.docs().get(0).getField(NestedPathFieldMapper.NAME).stringValue());
        assertEquals("value1", result.docs().get(0).getField("foo.bar").binaryValue().utf8ToString());
        
        assertNotNull(result.docs().get(1).getField(IdFieldMapper.NAME));
        assertEquals(Uid.encodeId("1"), result.docs().get(1).getField(IdFieldMapper.NAME).binaryValue());
        assertEquals(StringField.TYPE_STORED, result.docs().get(1).getField(IdFieldMapper.NAME).fieldType());
        assertNull(result.docs().get(1).getField(NestedPathFieldMapper.NAME));
        assertEquals("value2", result.docs().get(1).getField("baz").binaryValue().utf8ToString());
    }
    public void testPropagateDynamicWithExistingMapper() throws Exception {
        DocumentMapper mapper = createDocumentMapper(topMapping(b -> {
            b.field("dynamic", false);
            b.startObject("properties");
            {
                b.startObject("foo");
                {
                    b.field("type", "object");
                    b.field("dynamic", true);
                    b.startObject("properties").endObject();
                }
                b.endObject();
            }
            b.endObject();
        }));
        ParsedDocument doc = mapper.parse(source(b -> b.startObject("foo").field("bar", "something").endObject()));
        assertNotNull(doc.dynamicMappingsUpdate());