Scaffold terraform-provider-vault-secrets-arrstack

Configure the arrstack Vault secrets engine (backend config + roles) from
terraform-vault, matching the schema declared in terraform-vault #127.

- Add terraform-plugin-framework provider (local name arrstack) authenticating
  to Vault/OpenBao via address + token (VAULT_ADDR/VAULT_TOKEN fallback).
- Add arrstack_secret_backend resource: mounts the engine and writes <mount>/config.
- Add arrstack_secret_backend_role resource: manages <mount>/roles/<name>.
- Add Vault client, conversions, unit tests, Makefile, woodpecker CI + tag
  release to artifactapi terraform-unkin, examples, and README.

examples/resources/arrstack_secret_backend/main.tf

@@ -0,0 +1,26 @@
+terraform {
+  required_providers {
+    arrstack = {
+      source = "artifactapi.k8s.syd1.au.unkin.net/terraform-unkin/vault-secrets-arrstack"
+    }
+  }
+}
+
+provider "arrstack" {
+  # address / token fall back to VAULT_ADDR / VAULT_TOKEN.
+}
+
+# Mount the arrstack secrets engine and seed it with the arrproxy admin token.
+resource "arrstack_secret_backend" "arrstack" {
+  path     = "arrstack"
+  base_url = "https://arrstack.unkin.net"
+
+  admin_token = var.arrproxy_admin_token # e.g. sourced from vault_kv_secret_v2
+
+  request_timeout_seconds = 30
+}
+
+variable "arrproxy_admin_token" {
+  type      = string
+  sensitive = true
+}