Add WriteOnly context_wo support for ephemeral resources

[mcanevet]

Description

Add support for a context_wo (write-only) attribute in VM instances to handle sensitive ephemeral data that should not be persisted in Terraform state.

Use Case

When using ephemeral providers (like Vault, AWS Secrets Manager, etc.) to inject sensitive credentials into VM context, these values should not be stored in the Terraform state file. The context_wo attribute would allow passing sensitive data to VMs while ensuring it's never persisted.

Example use case:

resource "opennebula_virtual_machine" "vm" {
  name = "secure-vm"
  
  context = {
    NETWORK  = "YES"
    HOSTNAME = "vm-01"
  }
  
  context_wo = {
    # These values are passed to the VM but not stored in state
    API_KEY  = data.vault_generic_secret.api_key.data["value"]
    PASSWORD = random_password.db_password.result
  }
}

Benefits

• Enhanced security: Sensitive values never written to state files
• Compliance: Meets requirements for handling secrets
• Compatibility: Works seamlessly with ephemeral/write-only providers

Implementation Requirements

• Upgrade terraform-plugin-sdk/v2 to a version supporting WriteOnly attributes
• Merge context_wo with context during VM operations
• Ensure context_wo values are never read from or persisted to state