feat: add cockpit-zfs-manager for zfs builds

[Eelviny]

Long time lurker, first time contributor. This PR adds https://github.com/45Drives/cockpit-zfs-manager to ZFS builds of ucore-minimal.

You may notice that this repo actually provides .rpm builds and I wanted to use that, however those builds have a hard dependency on ZnapZend making it not easily usable. I instead opted to pull the release tar. I'm unsure whether locking to a specific version in this way is the best and feedback would be appreciated here.

I've tested these changes by creating an rpm-ostree usroverlay and running the commands manually, but unsure how best to properly integration test.