ci: Allow auto-backporting using labels

Allows automatic backport of merged PRs
based on the labels assigned.

Same as is currently done in e.g. trustify-ui:
- https://github.com/trustification/trustify-ui/blob/main/.github/workflows/pr-closed.yaml

Author: queria

.github/workflows/pr-closed.yaml

@@ -0,0 +1,34 @@
+name: PR Closed
+
+on:
+  pull_request_target:
+    branches:
+      - main
+    types:
+      - closed
+  issue_comment:
+    types:
+      - created
+
+jobs:
+  cherry_pick_job:
+    permissions:
+      pull-requests: write
+      contents: write
+    # Only run when pull request is merged
+    # or when a comment starting with `/backport` is created by someone other than the
+    # https://github.com/apps/trustification-ci-bot bot user (user id: 199085543). Note that if you use your
+    # own PAT as `github_token`, that you should replace this id with yours.
+    # To get Github App user id we can do: 'curl -H "Authorization: Bearer token" -s https://api.github.com/users/trustification-ci-bot%5Bbot%5D'
+    if: >
+      (
+        github.event_name == 'pull_request_target' &&
+        github.event.pull_request.merged
+      ) || (
+        github.event_name == 'issue_comment' &&
+        github.event.issue.pull_request &&
+        github.event.comment.user.id != 199085543 &&
+        startsWith(github.event.comment.body, '/backport')
+      )
+    secrets: inherit
+    uses: trustification/release-tools/.github/workflows/backport.yaml@main