fix: crw 2.0 requires backing up non-default container inside a pod

[pb82]

Implement container backup that backs up every container inside a pod. This has become necessary since CRW 2.0 because the projects directory is no longer in the default container.

Verification steps:

1. Get a cluster with RHMI installed
2. In the redhat-rhmi-codeready-workspaces namespace edit the cronjob for creating PV backups: oc edit cronjobs codeready-pv-backup
3. Replace the backup image with quay.io/pb82/backup-container-image:latest
4. Make sure that a workspace pod in CRW exists. If not open CRW and create a workspace.
5. Launch a backup job manually: oc create job codeready-pv-backup-$(date +"%Y-%m-%d-%H%M%S") --from=cronjob/codeready-pv-backup -n redhat-rhmi-codeready-workspaces
6. Check the logs of the backup pod and make sure workspace files are actually backed up: oc logs <backup pod name>

[sergioifg94]

Code looks good to me, and verified it on a cluster
/lgtm