OCI Object Storage as an S3-compatible backup destination

## Summary

I would like to contribute documentation for using Oracle Cloud Infrastructure
(OCI) Object Storage as an S3-compatible backup destination with the Barman
Cloud CNPG-I plugin.

This is a documentation/configuration contribution. It does not add a native
OCI-specific provider implementation.

## What was validated

- Continuous WAL archiving
- On-demand physical backup
- Scheduled backups
- Backup object validation in OCI Object Storage
- Restore/bootstrap of a new CloudNativePG cluster from the stored backup
- Recovery readiness / WAL replay

## OCI configuration

- OCI Object Storage S3 Compatibility API
- Endpoint pattern:
`https://.compat.objectstorage..oci.customer-oci.com`
- OCI Customer Secret Key used as S3 access key and secret key
- Explicit OCI region configured for request signing
- Unique destination path per PostgreSQL cluster

## Validated versions

| Component | Validated version / image |
|---|---|
| CloudNativePG Operator | 1.29.1 |
| Operator image | `ghcr.io/cloudnative-pg/cloudnative-pg:1.29.1` |
| PostgreSQL | 18.3 |
| PostgreSQL image | `ghcr.io/cloudnative-pg/postgresql:18.3-system-trixie` |
| PostgreSQL build | `postgres (PostgreSQL) 18.3 (Debian 18.3-1.pgdg13+1)` |
| Barman Cloud Plugin | 0.12.0 |
| Plugin image | `ghcr.io/cloudnative-pg/plugin-barman-cloud:v0.12.0` |
| Detected cluster plugin | `barman-cloud.cloudnative-pg.io`, version `0.12.0` |

## Proposed documentation

I can submit a concise OCI Object Storage provider guide containing:

1. OCI prerequisites and IAM guidance
2. Kubernetes Secret example using placeholders
3. `ObjectStore` manifest
4. Cluster WAL-archiving configuration
5. On-demand and scheduled backup examples
6. Recovery example
7. OCI-specific troubleshooting and security notes

Reference implementation and test details:
https://medium.com/oracledevs/cloudnativepg-oci-object-storage-a-practical-backup-and-restore-guide-98d35c204b55

Could you please confirm whether an OCI Object Storage configuration guide would be welcome as part of the Barman Cloud Plugin documentation?

Commencez par examiner la structure existante de la documentation de Barman Cloud Plugin et la configuration OCI validée décrite dans cet issue. Ajoutez un guide concis du fournisseur couvrant les prérequis, des exemples de Secret et ObjectStore, l’archivage de WAL, les sauvegardes, la récupération, le dépannage et les remarques de sécurité ; le travail est considéré comme terminé lorsque le workflow documenté correspond aux scénarios validés listés.

Rédigé par le modèle d'indexation à partir du texte de l'issue.