ci: add slack announce on release

Author: Codelax

.github/workflows/announce.yaml

@@ -0,0 +1,45 @@
+on:
+  release:
+    types:
+      - published
+jobs:
+  slack:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        webhook: ${{ fromJSON(secrets.SLACK_WEBHOOKS) }}
+    steps:
+      - name: Get tag
+        id: get_tag
+        run: echo ::set-output name=TAG::${GITHUB_REF/refs\/tags\//}
+      - name: Get version
+        id: get_version
+        run: echo ::set-output name=VERSION::${TAG##v}
+        env:
+          TAG: ${{ steps.get_tag.outputs.TAG }}
+      - name: Prepare message
+        id: get_message
+        run: echo ::set-output name=MSG::"${{ secrets.SLACK_MESSAGE }}"
+        env:
+          VERSION: ${{ steps.get_version.outputs.VERSION }}
+          TAG: ${{ steps.get_tag.outputs.TAG }}
+          RELEASE_URL: "https://github.com/scaleway/terraform-provider-scaleway/releases/tag/${{ steps.get_tag.outputs.TAG }}"
+      - name: Announce on slack
+        id: announce_slack
+        run: |
+          curl -X POST -H 'Content-type: application/json' \
+          --data '{
+          	"blocks": [
+              {
+                "type": "section",
+                "text": {
+                    "type": "mrkdwn",
+                    "text": "'"${MESSAGE}"'"
+                  }
+              }
+            ]
+          }' \
+          ${WEBHOOK_URL}
+        env:
+          WEBHOOK_URL: ${{ matrix.webhook }}
+          MESSAGE: ${{ steps.get_message.outputs.MSG }}