Support downloading the 2nd or 3rd download link from a KNS product

This lets LookAndFeel packages depend on specific versions of a KNS product.
This patch adds the ability to specify which link to download with the ?linkid=3 query parameter.
Eg: kns://plasmoids.knsrc/api.kde-look.org/1160672?linkid=3

Testing:
alias knshandler='/usr/lib/x86_64-linux-gnu/libexec/kf5/kpackagehandlers/knshandler'

Works:
knshandler kns://plasmoids.knsrc/api.kde-look.org/1160672
knshandler kns://plasmoids.knsrc/api.kde-look.org/1160672?linkid=3

Errors:
knshandler kns://plasmoids.knsrc/api.kde-look.org/1160672?linkid=test

Installs first download link (since it wasn't specified):
knshandler kns://plasmoids.knsrc/api.kde-look.org/1160672?rawr=blarg

BUG: 385429

Differential Revision: https://phabricator.kde.org/D8636