Write support for WFS (WFS-T) in GDAL integration

The GDAL integration supports reading data from WFS services by using the corresponding GDAL WFS driver. Here, I am using an XML descriptor file, since my Geoserver instance is behind basic auth,

```
$ cat my_wfs_descriptor_file.xml

http://localhost/geoserver/sf/ows?service=WFS
BASIC
admin:geoserver

```

Reading data works fine:

```
create table test as select * from ST_Read('my_wfs_descriptor_file.xml', allowed_drivers=ARRAY['WFS']);

select * from test limit 10;
```

However, writing back changes does not seem to work:

```
update test2 set str1 = 'TEST' where gml_id = 'archsites.12';

COPY test TO 'my_wfs_descriptor_file.xml' WITH (FORMAT gdal, DRIVER 'WFS');
Not implemented Error:
GDAL Error (6): GDALDriver::Create() ... no create method implemented for this format.
```

Is writing back to a WFS service not supported? The GDAL WFS driver itself states that it supports writing via WFS-T.