ci: add stale PR workflow (#169)

Adds the stale PR workflow that we also have in
https://github.com/substrait-io/substrait to substrait-python.

Signed-off-by: Niels Pardon <par@zurich.ibm.com>

Author: nielspardon

.github/workflows/stale.yml

@@ -0,0 +1,24 @@
+name: 'Close stale PRs'
+on:
+  schedule:
+    - cron: '0 0 * * *'
+  workflow_dispatch:
+
+permissions:
+  pull-requests: write
+
+jobs:
+  stale:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/stale@v10
+        with:
+          repo-token: ${{ secrets.GITHUB_TOKEN }}
+          days-before-issue-stale: -1 # don't operate on issues
+          stale-pr-message: |
+            This PR has been automatically marked as stale because it has not had
+            recent activity. It will be closed in 7 days if no further activity occurs.
+
+          close-pr-message: |
+            This PR has been automatically closed due to inactivity.
+            If you believe this was closed in error, please reopen it.