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.

.woodpecker/build.yml

@@ -0,0 +1,18 @@
+when:
+  - event: pull_request
+
+steps:
+  - name: build
+    image: golang:1.25
+    commands:
+      - make build
+    backend_options:
+      kubernetes:
+        serviceAccountName: default
+        resources:
+          requests:
+            memory: 512Mi
+            cpu: 1
+          limits:
+            memory: 2Gi
+            cpu: 2