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?

Start by reviewing the existing Barman Cloud Plugin documentation structure and the validated OCI configuration described in this issue. Add a concise provider guide covering prerequisites, Secret and ObjectStore examples, WAL archiving, backups, recovery, troubleshooting, and security notes; done means the documented workflow matches the listed validated scenarios.

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