Backup to different object store

Hi,
first of all thank you for the amazing work!

I might have a bit of an odd use case:
Currently I have two S3 compatible object stores. One onsite and one for offsite backups. Because the connection to the offsite one can be quite slow (this is a homelab setup), I didn't want to send WALs to the offsite S3 but just baseline backups every so often.

From what I can tell the BackupServiceImplementation currently just uses the barmanObjectKey from the cluster definition and ignores any parameters specified directly on the Backup or BackupSchedule CRD.

Would it be possible to support overriding the object store in the backup crd like this?

```
apiVersion: postgresql.cnpg.io/v1
kind: Backup
metadata:
name: backup-example
spec:
cluster:
name: pg-nextcloud
method: plugin
pluginConfiguration:
name: barman-cloud.cloudnative-pg.io
parameters:
barmanObjectName: anotherObjectStore

```

まず、BackupServiceImplementationと、issueで説明されているBackupおよびBackupSchedule CRDの処理を読みます。barmanObjectKeyがpluginConfigurationのパラメーターに対してどのように選択されるかを追跡し、次に既存のBackupテストを特定して、明示的に設定されたbarmanObjectNameがBackupリソースとBackupScheduleリソースの両方でクラスタのオブジェクトストアを上書きすることを確認します。

索引モデルが issue の本文から書いたものです。