Support Cluster Scoped ObjectStore in CNPG clusters

Currently, when configuring the Barman Cloud plugin in a Cluster resource, the barmanObjectName parameter only allows referencing ObjectStore configurations from the same namespace as the CNPG Cluster.

I would like to request support for cross-namespace references so that we can define shared ObjectStore configurations centrally (e.g., in a cnpg-system or infrastructure namespace), and reuse them across multiple database clusters in different namespaces.

Current Behavior:

```yaml
spec:
plugins:
- name: barman-cloud.cloudnative-pg.io
isWALArchiver: true
parameters:
barmanObjectName: minio-store # must exist in same namespace
```

This configuration requires a duplicate copy of minio-store in each namespace that has a Cluster.

Requested Behavior:
Allow for something like the following:

```yaml
spec:
plugins:
- name: barman-cloud.cloudnative-pg.io
isWALArchiver: true
parameters:
barmanObjectName: minio-store
barmanObjectNamespace: cnpg-system # new field to support cross-namespace
```

Use Case / Motivation:

Centralize storage credentials and reduce secret duplication across environments.

Aligns with GitOps, multi-tenant, and RBAC best practices.

Improves DRY (Don't Repeat Yourself) and credential management in production-grade Kubernetes platforms.

Additional Context:
Similar cross-namespace reference patterns are used in projects like External Secrets Operator, cert-manager, and ArgoCD. This change would help platform teams standardize and secure backup configuration for PostgreSQL clusters across many namespaces.

ファイルやテストは指定されていません。まずリポジトリで barmanObjectName と、ObjectStore 参照に対する namespace の検索または検証を検索し、その後 Cluster のプラグインパラメータがどのように解決されるかを追跡します。提案された namespace フィールドを通じて、Cluster が別の namespace にある ObjectStore を参照でき、適切な検証と namespace をまたぐ動作のテストがあることが完了条件です。

索引モデルが issue の本文から書いたものです。