CI: Notify slack channel on releases (linode#469)

* add release-notify-slack workflow

* add release-notify-slack workflow

* add workflow dispatch

* gha test 1

* gha test 2

* gha test 3

* gha test 4

* gha test 5

* fix payload syntax

* change environment variable name

* update slack payload for gh test

* update slack payload to include link to release notes

Author: ykim-akamai

.github/workflows/release-notify-slack.yml

@@ -0,0 +1,30 @@
+name: Notify Dev DX Channel on Release
+on:
+  release:
+    types: [published]
+  workflow_dispatch: null
+
+jobs:
+  notify:
+    if: github.repository == 'linode/linode_api4-python'
+    runs-on: ubuntu-latest
+    steps:
+      - name: Notify Slack - Main Message
+        id: main_message
+        uses: slackapi/slack-github-action@v1.27.0
+        with:
+          channel-id: ${{ secrets.DEV_DX_SLACK_CHANNEL_ID }}
+          payload: |
+            {
+              "blocks": [
+                {
+                  "type": "section",
+                  "text": {
+                    "type": "mrkdwn",
+                    "text": "*New Release Published: _linode_api4-python_ <${{ github.event.release.html_url }}|${{ github.event.release.tag_name }}> is now live!* :tada:"
+                  }
+                }
+              ]
+            }
+        env:
+          SLACK_BOT_TOKEN: ${{ secrets.SLACK_BOT_TOKEN }}