Add reusable PR Slack notification workflow

.github/workflows/pr-slack-notification.yml

@@ -0,0 +1,42 @@
+name: PR Slack Notification
+
+on:
+  workflow_call:
+    secrets:
+      slack_webhook_url:
+        required: true
+
+permissions: {}
+
+jobs:
+  notify:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Post to Slack
+        env:
+          SLACK_WEBHOOK_URL: ${{ secrets.slack_webhook_url }}
+          PR_AUTHOR: ${{ github.event.pull_request.user.login }}
+          PR_NUMBER: ${{ github.event.pull_request.number }}
+          PR_TITLE: ${{ github.event.pull_request.title }}
+          PR_URL: ${{ github.event.pull_request.html_url }}
+          REPOSITORY: ${{ github.repository }}
+        run: |
+          set -o pipefail
+
+          jq -n \
+            --arg author "$PR_AUTHOR" \
+            --arg number "$PR_NUMBER" \
+            --arg repository "$REPOSITORY" \
+            --arg title "$PR_TITLE" \
+            --arg url "$PR_URL" \
+            '{
+              text: "Pull request notification",
+              blocks: [
+                {type: "section", text: {type: "mrkdwn", text: ("*Pull request:* <" + $url + "|" + $repository + " #" + $number + ">")}},
+                {type: "section", text: {type: "plain_text", text: ($title + " — by @" + $author), emoji: true}}
+              ]
+            }' |
+            curl --fail-with-body --silent --show-error \
+              --header 'Content-type: application/json' \
+              --data @- \
+              "$SLACK_WEBHOOK_URL"