Scaffold the bind-tsig secrets engine

A Vault/OpenBao secrets engine that manages BIND TSIG keys via the
bind-operator companion API (Vault -> HTTP API -> BindTSIGKey CRs).

- backend + cmd entry point (plugin.ServeMultiplex), modelled on
  vault-plugin-secrets-litellm
- config path: companion API url/token/tls + defaults
- static-roles/static-creds: stable named key with managed rotation
- roles/creds: dynamic, lease-bound keys (revoke deletes the CR)
- tsig_key secret type with revoke/renew
- HTTP client for the companion API contract (/v1/keys CRUD + rotate)
- Makefile, Woodpecker CI (pre-commit/build/test + tag release RPMs),
  nfpm packaging (vault + openbao flavours)
- e2e: mock companion API + Vault + OpenBao in docker-compose, full
  lifecycle per engine; unit tests for the dynamic + static flows

.gitignore

@@ -0,0 +1,5 @@
+/dist/
+/vault-plugin-secrets-bind-tsig
+*.out
+*.test
+.env