Support dynamic metadata request on PROPFIND requests

[artonge]

Depends on #40761
Part of #40676

Changes

• Delete old metadata API files
• Add support for getting metadata in PROPFIND request
• Add support for setting metadata in PROPPATCH request

Test requests

curl 'https://nextcloud_server1.test/remote.php/dav/files/admin/img.jpg' \
  --insecure \
  --user admin:password \
  --request PROPFIND \
  --data '<?xml version="1.0" encoding="UTF-8"?>
    <d:propfind xmlns:d="DAV:" xmlns:oc="http://owncloud.org/ns" xmlns:nc="http://nextcloud.org/ns">
      <d:prop>
        <d:getlastmodified/>
        <d:getcontentlength/>
        <d:getcontenttype/>
        <nc:metadata-files-test/>
      </d:prop>
    </d:propfind>' | xq

curl 'https://nextcloud_server1.test/remote.php/dav/files/admin/img.jpg' \
  --insecure \
  --user admin:password \
  --request PROPPATCH \
  --data '<?xml version="1.0" encoding="UTF-8"?>
    <d:propertyupdate xmlns:d="DAV:" xmlns:oc="http://owncloud.org/ns" xmlns:nc="http://nextcloud.org/ns">
      <d:set>
            <d:prop>
              <nc:metadata-files-test>2</nc:metadata-files-test>
            </d:prop>
      </d:set>
    </d:propertyupdate>' | xq

[skjnldsv]

Muuuch cleaner!
Requires a documentation update regarding this API
https://github.com/nextcloud/documentation/blob/master/developer_manual/app_publishing_maintenance/app_upgrade_guide/upgrade_to_28.rst