use speakeasy slack notifier for ci failures (#29)

Author: atonks2

.github/workflows/sdk_generation.yaml

@@ -28,3 +28,4 @@ jobs:
       github_access_token: ${{ secrets.GITHUB_TOKEN }}
       npm_token: ${{ secrets.NPM_TOKEN }}
       speakeasy_api_key: ${{ secrets.SPEAKEASY_API_KEY }}
+      slack_webhook_url: ${{ secrets.SLACK_WEBHOOK }}