fix: Allow getting custom resources from all namespaces

[banjoh]

This will allow running kubectl get custom-resource -A

Before

[evans] $ k get backups -A
No resources found

After

[evans] $ k get backups -A
NAMESPACE   NAME             AGE
velero      instance-pz2c8   20d
velero      instance-6rxr7   20d
velero      instance-qqzzv   19d
velero      instance-c2964   18d
velero      instance-9fffl   17d
velero      instance-4vjzw   16d
velero      instance-mffms   15d
velero      instance-khdtt   14d
velero      instance-b69wr   13d
velero      instance-rnnfm   12d