Add non-namespaced SnapshotSchedules

[timstoop]

Describe the feature you'd like to have.
I would like to be able to deploy a single (or several) SnapshotSchedules cluster wide and have them operate on all namespaces. In most cases that we see, it makes more sense to make a single SnapshotSchedule available cluster-wide and use labels (or possible annotations) to have them applied to PVCs.

What is the value to the end user? (why is it a priority?)
Easier to add existing snapshot schedules when you prefer spreading out your applications over multiple namespaces. Also easier to have a single "enforced" snapshot schedule that should be adhered to. Or to change a schedule if it is warranted.

How will we know we have a good solution? (acceptance criteria)
When a snapshot schedule only needs to be defined once on a cluster.

Additional context
I would recommend calling it something like ClusterSnapshotSchedule, in line with upstream resources like Role and ClusterRole or resources provided by commonly used third parties, like Issuer and ClusterIssuer.

[bo0tzz]

I'd like to have this option as well. My usecase specifically would be to apply a schedule indiscriminately to all PVCs of a certain StorageClass.