freebsd: add zfs to the default plugins

[samuelkarp]

Note: this PR will remain in draft until containerd/zfs#43 #5380 is merged.

$ sudo ctr run \
    --snapshotter=native \
    --runtime wtf.sbk.runj.v1 \
    --rm \
    public.ecr.aws/samuelkarp/freebsd:12.1-RELEASE test-native mount
/var/lib/containerd/io.containerd.snapshotter.v1.native/snapshots/153 on / (nullfs, local)
$ sudo ctr run \
    --snapshotter=zfs \
    --runtime wtf.sbk.runj.v1 \
    --rm \
    public.ecr.aws/samuelkarp/freebsd:12.1-RELEASE test-zfs mount   
zroot/containerd/4 on / (zfs, local, noatime, nfsv4acls)

[k8s-ci-robot]

Skipping CI for Draft Pull Request.
If you want CI signal for your change, please convert it to an actual PR.
You can still manually trigger a test run with /test all

[theopenlab-ci]

Build succeeded.

• containerd-build-arm64 : SUCCESS in 5m 57s (non-voting)