use efficient tag retrieval on DAV report request

use efficient tag retrieval on DAV report request

- uses DAV search approach against valid files joined by systemtag selector
- reduced table join for tag/systemtag search
- supports pagination
- no changes to the output formats or similar

Example request body:

<?xml version="1.0"?>
<oc:filter-files xmlns:d="DAV:" xmlns:oc="http://owncloud.org/ns" xmlns:nc="http://nextcloud.org/ns" xmlns:ocs="http://open-collaboration-services.org/ns">
  <d:prop>
    <d:getcontentlength/>
    <d:getcontenttype/>
    <d:getetag/>
    <d:getlastmodified/>
    <d:resourcetype/>
    <nc:face-detections/>
    <nc:file-metadata-size/>
    <nc:has-preview/>
    <nc:realpath/>
    <oc:favorite/>
    <oc:fileid/>
    <oc:permissions/>
    <nc:nbItems/>
  </d:prop>
  <oc:filter-rules>
    <oc:systemtag>32</oc:systemtag>
  </oc:filter-rules>
  <d:limit>
    <d:nresults>50</d:nresults>
    <nc:firstresult>0</nc:firstresult>
  </d:limit>
</oc:filter-files>

Signed-off-by: Arthur Schiwon <blizzz@arthur-schiwon.de>