Using async config can result in the loss of the WAL files

[l00ptr]

We use the '/controller/wal-spool' directory to enable pgbackrest to store WAL files when 'async' is set to 'true'. As this directory is ephemeral and not linked to a persistent volume (PV), it's easy to imagine situations where we could temporarily lose the WAL files stored here if something goes wrong with the pgbackrest sidecar. Perhaps we should implement a way to add a dedicated volume when using the 'async' configuration.