Allow to trigger release manually (#1359)

contrib/perform-release.sh

@@ -0,0 +1,18 @@
+#!/bin/sh -e
+
+# Manually trigger a release of project "Piper".
+# Usually we do release on a schedule, but sometimes you might need to trigger a release.
+# Invoke this script with PIPER_RELEASE_TOKEN set to your personal access token for GitHub with 'repo' scope.
+# This script is based on https://goobar.io/2019/12/07/manually-trigger-a-github-actions-workflow/
+
+if [ -z "$PIPER_RELEASE_TOKEN" ]
+then
+    echo "Required variable PIPER_RELEASE_TOKEN is not set, please set a personal access token for GitHub with 'repo' scope."
+    exit 1
+fi
+
+curl -H "Accept: application/vnd.github.everest-preview+json" \
+    -H "Authorization: token ${PIPER_RELEASE_TOKEN}" \
+    --request POST \
+    --data '{"event_type": "perform-release"}' \
+    https://api.github.com/repos/SAP/jenkins-library/dispatches