Add internal fileid to webdav response

[PVince81]

Introduce a new property "oc:fileid" to return the internal file id.

This is because the original "oc:id" property is a compound and it is
not possible to extract the real id without knowing the instance id. The
instance id is not available to external clients.

Note: this is required for the new web UI impl that uses Webdav: #16902

To test:

1. Create a folder "test"
2. Save the following into "propfind.txt"

<?xml version="1.0"?>
<a:propfind xmlns:a="DAV:" xmlns:oc="http://owncloud.org/ns">
        <a:prop><oc:id/></a:prop>
        <a:prop><oc:fileid/></a:prop>
</a:propfind>

1. curl -X PROPFIND -H "Content-Type: text/xml" --data-binary "@propfind.txt" http://root:admin@localhost/owncloud/remote.php/webdav/test | xmllint --format -

This PR will output two additional properties, when requested as above:

<d:response>
    <d:href>/owncloud/remote.php/webdav/test/</d:href>
    <d:propstat>
      <d:prop>
        <oc:id>00000003ockbfur1449k</oc:id>
        <oc:fileid>3</oc:fileid>
      </d:prop>
      <d:status>HTTP/1.1 200 OK</d:status>
    </d:propstat>
  </d:response>

Please review @DeepDiver1975 @nickvergessen @icewind1991 @rullzer

I'm also open to better suggestions. Another alternative is to expose the "instanceid" through some API to make it possible to extract the file id from "oc:id".

[DeepDiver1975]

👍

[icewind1991]

👍 looks good