Add GCS and Azure Blob storage backends

[lanycrost]

Summary

Add storage driver support for Google Cloud Storage and Azure Blob Storage alongside the existing S3-compatible backend.

Problem

Currently only S3-compatible backends are supported (SeaweedFS, MinIO, Garage, AWS S3). Users on GCP or Azure must run an S3-compatible proxy.

Proposed change

• Add a GCS driver using the native GCS SDK
• Add an Azure Blob driver using the native Azure SDK
• Maintain the existing S3 interface as default
• Configuration via the same storage environment variables / operator config

References

• Roadmap: https://bubustack.io/docs/community/roadmap
• Payloads docs: https://bubustack.io/docs/runtime/payloads