Add Oracle Cloud Infrastructure (OCI) Object Storage backend

[harsha-yuvaraj]

What this PR does:

Adds Oracle Cloud Infrastructure (OCI) Object Storage as a storage backend for blocks, ruler, and alertmanager, wrapping the OCI provider already shipped by Thanos objstore. Select it with backend: oci and configure via -<prefix>.oci.* flags.

• pkg/storage/bucket/oci/config.go — config struct + flags (auth providers: default, instance-principal, raw, oke-workload-identity)
• pkg/storage/bucket/oci/bucket_client.go — thin wrapper over objstore/providers/oci
• pkg/storage/bucket/client.go — backend registration (constant, SupportedBackends, config field, NewClient case)
• Vendored github.com/oracle/oci-go-sdk/v65

Because ruler/alertmanager/blocks validate against bucket.SupportedBackends, OCI is available to all three with no extra wiring. Retries are enabled by default (max_request_retries: 3, request_retry_interval: 10s) for parity with the other backends; the provider performs no retries when these are 0/1.

Which issue(s) this PR fixes:

Fixes #7593

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated
• docs/configuration/v1-guarantees.md updated (OCI listed as experimental)

[krunaljain]

Should this PR be broken down?

[CharlieTLe]

Hi @harsha-yuvaraj, thanks for working on this. Were you able to get this working in a real OCI setup?

[harsha-yuvaraj]

Hi @CharlieTLe, yes, validated end-to-end against a real OCI Object Storage bucket (Always Free tier), using the default provider (~/.oci/config).

I ran the backend's full read/write cycle through NewBucketClient: Upload → Exists → Get (with content verification) → Iter → Delete, and every operation succeeded:

=== RUN   TestOCILive
level=info msg="creating OCI client" provider=default
level=info msg="getting namespace, it might take some time"
level=debug msg="Oracle Cloud Infrastructure tenancy namespace: <redacted>"
    live_test.go:51: Upload...
    live_test.go:54: Exists (expect true)...
    live_test.go:59: Get + verify content...
    live_test.go:67: Iter (expect to find the object)...
level=debug NumberOfObjects=1
    live_test.go:77: Delete...
    live_test.go:84: ALL OCI LIVE OPERATIONS PASSED
--- PASS: TestOCILive (2.51s)
PASS
ok      github.com/cortexproject/cortex/pkg/storage/bucket/oci  3.443s

The check drove the same objstore.Bucket operations Cortex uses in the blocks/ruler/alertmanager paths.