Allow DAV Object properties

The current implementation only saves them as string. It seems they can
be more complex than that, and that objects were saved directly.

You may find such objects saved in some production databases by
executing:
```sql
SELECT * from oc_properties where propertyvalue = 'Object';
```

This commit adds a repair job to clean all of these "broken" properties
values, adds a new database column to save the type of the property, and
handles converting from and to correct values.

Implementation is very similar to SabreDAV's own PDO backend: https://github.com/nextcloud/3rdparty/blob/4921806dfb1c5c309eac60195ed34e2749baf3c1/sabre/dav/lib/DAV/PropertyStorage/Backend/PDO.php



Signed-off-by: Thomas Citharel <tcit@tcit.fr>