Add GPG/OpenPGP secrets engine

Provide a transit-style Vault/OpenBao secrets engine whose key material is
OpenPGP, so services can sign/verify/encrypt/decrypt with GPG keys that never
leave the barrier — and so tools like pass can encrypt to an exported public
key while delegating decryption back to Vault.

- Add versioned key management (keys/<name> CRUD+list, config, rotate, import)
  with private material seal-wrapped under key/ and per-key locking.
- Add sign/verify (detached OpenPGP) and encrypt/decrypt paths; decrypt
  auto-detects armored vs raw-binary ciphertext (what pass/gpg write).
- Add export/<public-key|private-key>/<name>; public always exportable,
  private only when the key is marked exportable.
- Use ProtonMail go-crypto for OpenPGP; support rsa-2048/3072/4096 and ed25519.
- Clone the sibling plugin's build/packaging/CI: dual-target RPMs
  (vault + openbao plugin dirs), Woodpecker PR/release pipelines, and a
  Vault+OpenBao e2e harness. Unit tests include real gpg and pass interop.

.woodpecker/test.yml

@@ -0,0 +1,33 @@
+when:
+  - event: pull_request
+
+steps:
+  - name: lint
+    image: golang:1.25
+    commands:
+      - make lint
+    backend_options:
+      kubernetes:
+        serviceAccountName: default
+        resources:
+          requests:
+            memory: 512Mi
+            cpu: 1
+          limits:
+            memory: 2Gi
+            cpu: 2
+
+  - name: test
+    image: golang:1.25
+    commands:
+      - make test
+    backend_options:
+      kubernetes:
+        serviceAccountName: default
+        resources:
+          requests:
+            memory: 512Mi
+            cpu: 1
+          limits:
+            memory: 2Gi
+            cpu: 2