ci: add scheduled run of GHA CI

This is to ensure that our CI is not rotting away even if there are no
new PRs or merges. This is especially useful for release branches
which tend to cease working over time due to some external reasons.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

.github/workflows/scheduled.yml

@@ -0,0 +1,35 @@
+# This enables periodical execution of CI jobs in branches we maintain.
+#
+# CI jobs are triggered through here (instead of adding "schedule:" to the
+# appropriate files) because scheduled jobs are only run on the main branch.
+# In other words, it's a way to run periodical CI for other branches.
+
+name: scheduled
+on:
+  schedule:
+    # Runs at 00:00 UTC every Sunday, Tuesday, Thursday.
+    - cron: '0 0 * * 0,2,4'
+  workflow_dispatch:
+permissions:
+  contents: read
+  actions: write
+
+jobs:
+  trigger-workflow:
+    strategy:
+      matrix:
+        branch: ["main", "release-1.3"]
+        wf_id: ["validate.yml", "test.yml"]
+    runs-on: ubuntu-latest
+    steps:
+      - name: Trigger ${{ matrix.wf_id }} workflow on ${{ matrix.branch}} branch
+        uses: actions/github-script@v7
+        with:
+          github-token: ${{ secrets.GITHUB_TOKEN }}
+          script: |
+            await github.rest.actions.createWorkflowDispatch({
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              workflow_id: '${{ matrix.wf_id }}',
+              ref: '${{ matrix.branch }}'
+            });