Add support for meta-path-for-user propfind

[kulmann]

This PR adds support for the meta-path-for-user prop in propfinds to the /dav/meta endpoint. This is needed in oC Web for following situations:
a) when no resource path but only its id is given in the browser URL, fetch the path to make a propfind afterwards
b) when a resource path and id is given in the browser URL but resolves to an outdated location (e.g. resource has been moved), fetch new path to make another propfind with the correct path afterwards

Based on outdated/closed #2321 and #1604

Spec can be found here: https://doc.owncloud.com/server/next/developer_manual/webdav_api/meta.html

An example request:
curl -u einstein:relativity --insecure -X PROPFIND -H "Depth: 0" -H "Content-Type: text/xml" --data "@meta-files.xml" 'https://localhost:9200/remote.php/dav/meta/ddc2004c-0977-11eb-9d3f-a793888cd0f8!aa07d86a-f564-4e26-9e36-632ef63df9f0' | xmllint --format -

with content of meta-files.xml being:

<?xml version="1.0"?>
<a:propfind xmlns:a="DAV:" xmlns:oc="http://owncloud.org/ns">
    <a:prop>
        <oc:meta-path-for-user/>
    </a:prop>
</a:propfind>

[phil-davis]

Now we just have: https://drone.cernbox.cern.ch/cs3org/reva/6755/16/6

  Scenario: User cannot access meta folder of a file which does not exist                                                                                                             # /drone/src/tmp/testrunner/tests/acceptance/features/apiVersions/fileVersions.feature:388
    Given user "Brian" has been created with default attributes and without skeleton files                                                                                            # FeatureContext::userHasBeenCreatedWithDefaultAttributesAndWithoutSkeletonFiles()
When user "Brian" sends HTTP method "PROPFIND" to URL "/remote.php/dav/meta/MTI4NGQyMzgtYWE5Mi00MmNlLWJkYzQtMGIwMDAwMDA5MTU2OjhjY2QyNzUxLTkwYTQtNDBmMi1iOWYzLTYxZWRmODQ0MjFmNA==" # FeatureContext::userSendsHTTPMethodToUrl()
--
<span class="ansi-red-fg">      cURL error 52: Empty reply from server (see https://curl.haxx.se/libcurl/c/libcurl-errors.html) for http://revad-services:20080/remote.php/dav/meta/MTI4NGQyMzgtYWE5Mi00MmNlLWJkYzQtMGIwMDAwMDA5MTU2OjhjY2QyNzUxLTkwYTQtNDBmMi1iOWYzLTYxZWRmODQ0MjFmNA== (GuzzleHttp\Exception\ConnectException)</span>

Maybe the server code is "completely silent" if the request does not match any resource?
(and previously this passed - the server must have returned something - a 404 not found)