Add Slack notifications for PRs and issues (#1456)

Adds a Slack notifier URL workflow (copied from Pytorch connector)

### Does this change impact existing behavior?

No

### Does this change need a changelog entry? Does it require a version
change?

No

---

By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 license and I agree to the terms of
the [Developer Certificate of Origin
(DCO)](https://developercertificate.org/).

---------

Signed-off-by: Simon Beal <[email protected]>

Author: muddyfish

.github/workflows/notify_slack.yml

@@ -0,0 +1,35 @@
+name: Slack Notifications
+
+on:
+  issues:
+    types: [opened, reopened, edited]
+  pull_request_target:
+    types: [opened, reopened, synchronize]
+
+jobs:
+  notify:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Send issue notification to Slack
+        if: github.event_name == 'issues'
+        uses: slackapi/[email protected]
+        with:
+          webhook: ${{ secrets.SLACK_WEBHOOK_URL_ISSUE }}
+          webhook-type: incoming-webhook
+          payload: |
+            {
+              "action": "${{ github.event.action }}",
+              "issue_url": "${{ github.event.issue.html_url }}"
+            }
+
+      - name: Send pull request notification to Slack
+        if: github.event_name == 'pull_request_target'
+        uses: slackapi/[email protected]
+        with:
+          webhook: ${{ secrets.SLACK_WEBHOOK_URL_PR }}
+          webhook-type: incoming-webhook
+          payload: |
+            {
+              "action": "${{ github.event.action }}",
+              "url": "${{ github.event.pull_request.html_url }}"
+            }