Add WKD export functionality to keyringctl

We should directly implement the export of a WKD directory on push to default branch using sq wkd generate. This allows for the shortest turnaround time and removes the necessity for https://gitlab.archlinux.org/archlinux/wkd.

In regards to the implementation of this feature, the following needs to be taken into account:

• we would like to generate WKD for all currently used domains: archlinux.org and master-key.archlinux.org
• we would like to generate WKD for all keys (main keys, packager keys and staff keys)
• sq wkd generate can only be fed with keyrings containing keys that have matching UIDs for the target domain (else generating fails!), which will require us to create combined temporary keyring files for use in the export or export keys one-by-one if they match the given target domain