Build an OpenBao RPM alongside the Vault one

The plugin runs under both Vault and OpenBao, but nfpm only produced a
single RPM that installs into /opt/vault-plugins. OpenBao hosts expect
their plugins under /opt/openbao-plugins, so a second package is needed.

- Parameterize nfpm.yaml with PACKAGE_NAME, PACKAGE_PLUGIN_DIR, and
  PACKAGE_PREINSTALL so one config renders per target server
- Replace the static preinstall.sh with preinstall.sh.tmpl that
  mkdir -p's the flavour's plugin directory
- Build two RPMs in build-rpm.sh via a build_flavor helper:
  vault-plugin-secrets-litellm -> /opt/vault-plugins and
  openbao-plugin-secrets-litellm -> /opt/openbao-plugins

packaging/nfpm.yaml

@@ -1,6 +1,8 @@
 ---
 # nfpm config for building the vault-plugin-secrets-litellm RPM.
 # Rendered through envsubst (see scripts/build-rpm.sh) then fed to `nfpm pkg`.
+# Built once per target server (Vault, OpenBao); PACKAGE_NAME and
+# PACKAGE_PLUGIN_DIR vary per flavour.
 
 name: ${PACKAGE_NAME}
 version: ${PACKAGE_VERSION}
@@ -18,19 +20,19 @@ license: ${PACKAGE_LICENSE}
 disable_globbing: false
 
 replaces:
-  - vault-plugin-secrets-litellm
+  - ${PACKAGE_NAME}
 provides:
-  - vault-plugin-secrets-litellm
+  - ${PACKAGE_NAME}
 
-# Install the plugin binary into the Vault/OpenBao plugin directory. Point the
-# server's plugin_directory at /opt/vault-plugins to pick it up.
+# Install the plugin binary into the server's plugin directory. Point the
+# server's plugin_directory at PACKAGE_PLUGIN_DIR to pick it up.
 contents:
   - src: dist/vault-plugin-secrets-litellm
-    dst: /opt/vault-plugins/vault-plugin-secrets-litellm
+    dst: ${PACKAGE_PLUGIN_DIR}/vault-plugin-secrets-litellm
     file_info:
       mode: 0755
       owner: root
       group: root
 
 scripts:
-  preinstall: packaging/scripts/preinstall.sh
+  preinstall: ${PACKAGE_PREINSTALL}

packaging/scripts/preinstall.sh

@@ -1,3 +0,0 @@
-#!/usr/bin/env bash
-# Ensure the plugin directory exists before the binary is laid down.
-mkdir -p /opt/vault-plugins

packaging/scripts/preinstall.sh.tmpl

@@ -0,0 +1,4 @@
+#!/usr/bin/env bash
+# Ensure the plugin directory exists before the binary is laid down.
+# Rendered per flavour via envsubst (see scripts/build-rpm.sh).
+mkdir -p ${PACKAGE_PLUGIN_DIR}