Make <oc:privatelink> property available in PROPFIND

[michaelstingl]

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

ownCloud desktop sync client with oC10 can open every file or folder in the web UI. Therefore it PROPFIND with 'Depth: 0' to get the <oc:privatelink> property. This doesn't work with oCIS yet.

Describe the solution you'd like

Make <oc:privatelink> property available in PROPFIND, similar to oC10.

Desktop sync clients respect the privateLinks capability, so it must be added there too. Then it will "just work". (owncloud/client#8998)

Describe alternatives you've considered

I thought about QuickLink creation, but not every user has the permission to do so.

Additional context

oC10 PROPFIND

% curl -u test:test \
 -H 'Depth: 0' \
 -X PROPFIND https://demo.owncloud.org/remote.php/dav/files/test/Photos -d '<?xml version="1.0" encoding="utf-8"?><d:propfind xmlns:d="DAV:"><d:prop><privatelink xmlns="http://owncloud.org/ns"/></d:prop>M</d:propfind>' | xmllint --format - 
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100   494  100   353  100   141   1381    551 --:--:-- --:--:-- --:--:--  1991
<?xml version="1.0"?>
<d:multistatus xmlns:d="DAV:" xmlns:s="http://sabredav.org/ns" xmlns:oc="http://owncloud.org/ns">
  <d:response>
    <d:href>/remote.php/dav/files/test/Photos/</d:href>
    <d:propstat>
      <d:prop>
        <oc:privatelink>https://demo.owncloud.org/f/20</oc:privatelink>
      </d:prop>
      <d:status>HTTP/1.1 200 OK</d:status>
    </d:propstat>
  </d:response>
</d:multistatus>

oC10 capabilities

 % curl -u test:test 'https://demo.owncloud.org/ocs/v1.php/cloud/capabilities?format=json' | jq '.ocs.data.capabilities.files'
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  2167  100  2167    0     0   9962      0 --:--:-- --:--:-- --:--:-- 10221
{
  "privateLinks": true,
  "privateLinksDetailsParam": true,
  "bigfilechunking": true,
  "blacklisted_files": [
    ".htaccess"
  ],
  "blacklisted_files_regex": "\\.(part|filepart)$",
  "favorites": true,
  "file_locking_support": true,
  "file_locking_enable_file_action": false,
  "undelete": true,
  "versioning": true
}

@micbar @kulmann

[michaelstingl]

Separate PR to bring back the capability: #4599