Add Huawei Cloud OBS storage backend

[CharlieTLe]

Thanos objstore supports Huawei Cloud Object Storage (OBS provider). Add it as a Cortex storage backend for blocks, ruler, and alertmanager storage.

Implementation steps

1. Create pkg/storage/bucket/obs/config.go — Config struct with RegisterFlagsWithPrefix()
2. Create pkg/storage/bucket/obs/bucket_client.go — Thin wrapper around objstore/providers/obs
3. Register in pkg/storage/bucket/client.go — Add constant, config field, and NewClient() case
4. Vendor the OBS provider and Huawei Cloud SDK
5. Add unit tests and CHANGELOG entry