fix #5801: backup_manager: make api call on datastore update

fix #5801: backup_manager: make api call on datastore update

When updating the datastore config using `proxmox-backup-manager` we
need to make an api-call, because the api-route starts a tokio task to
update the proxy-cache and the client will kill the task if we don't
wait. With an api-call the tokio task will be executed on the api
process and runs in the background while the endpoint handler has
already returned.

Signed-off-by: Gabriel Goller <g.goller@proxmox.com>