`\OCA\DAV\DAV\CustomPropertiesBackend` should use the query builder or a QBMapper

[ChristophWurst]

Is your feature request related to a problem? Please describe.

The class \OCA\DAV\DAV\CustomPropertiesBackend builds manual SQLs for the most part.

Describe the solution you'd like

Migrate to the query builder or even offload the storage aspects to a dedicated QBMapper.

Describe alternatives you've considered

n/a

Additional context

• https://github.com/nextcloud/server/blob/master/apps/dav/lib/DAV/CustomPropertiesBackend.php