Add Tencent Cloud COS storage backend

[CharlieTLe]

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

Implementation steps

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