Implement Secret-based credential management

[Shahab96]

Currently, the operator only supports reading RustFS credentials (accesskey/secretkey) from environment variables. We need to add support for reading credentials from Kubernetes Secrets for better security and integration with secret management systems.

Current Behavior

• Credentials are extracted via get_tenant_credentials() in src/context.rs:187
• Only environment variables (accesskey, secretkey) are supported
• TODO comment exists at line 187: "Add support for reading credentials from Secrets"

Desired Behavior

Support multiple credential sources:

1. Kubernetes Secrets (referenced in Tenant spec)
2. Environment variables (existing behavior, for backward compatibility)
3. Proper validation and error handling for missing/invalid credentials

Implementation Considerations

• Add optional credentialsSecret field to Tenant spec
• Update get_tenant_credentials() to check Secret first, then fall back to env vars
• Add validation for credential format and requirements
• Update documentation and examples

Priority

High - Core Stability (from ROADMAP.md)

Related

• Referenced in: src/context.rs:187
• Part of: Core Stability roadmap phase