feat(storage_box): support write-only password attributes

[jfilter]

Closes #1299

Adds password_wo and password_wo_version to hcloud_storage_box and hcloud_storage_box_subaccount, so the Storage Box password never reaches the Terraform state. password becomes optional; exactly one of password and password_wo must be set (ExactlyOneOf), and password_wo requires password_wo_version.

A write-only value is null in plan and state, so it cannot be compared against either. A password reset is therefore triggered by incrementing password_wo_version — the same model the HashiCorp providers use (e.g. aws_db_instance.password_wo_version). Moving an existing configuration from password to password_wo drops the stored password from the state on the next apply.

Write-only arguments need Terraform 1.11 / OpenTofu 1.11 or later; the new acceptance tests skip below that, so the OpenTofu 1.10.x job in the matrix is unaffected.

• Schema, model and Create/Update paths for both resources
• Docs, examples and test templates updated (go generate is clean)
• Acceptance tests: create with password_wo, unchanged plan on same version, reset on incremented version, password/password_wo never written to state