CI: allow for self-approvals for maintainers (#7921)

.github/CODEOWNERS

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+.github/CODEOWNERS @keradus
+.github/workflows/self-approval.yml @keradus

.github/workflows/self-approval.yml

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+# Flow for self-approving the PRs, used by maintainers for cases aligned between them.
+# To trigger it, the maintainer needs to commit and push:
+# $ git commit -m "self-approval" --allow-empty
+
+name: Self Approval
+
+on:
+  - pull_request_target
+
+permissions:
+  pull-requests: write
+  contents: write
+
+jobs:
+  automate:
+    name: Automate
+    runs-on: ubuntu-latest
+    if: ${{ contains(fromJson('["julienfalque", "keradus", "keradus-ci", "kubawerlos", "localheinz", "Wirone"]'), github.actor) }}
+    env:
+      PR_URL: ${{github.event.pull_request.html_url}}
+      GITHUB_TOKEN: ${{secrets.GITHUB_TOKEN}}
+      MARKER: "self-approval"
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+        with:
+          ref: ${{ github.event.after }}
+      - name: Determine whether PR should be automatically approved
+        id: marker
+        run: |
+          msg=$(git show -s --format=%s ${{ github.event.after }})
+          echo $msg
+          indicator=$([[ $msg == $MARKER ]] && echo yes || echo no)
+          echo $indicator
+          echo "indicator_value=${indicator}" >> "$GITHUB_OUTPUT"
+      - name: Approve and enable auto-merge
+        if: steps.marker.outputs.indicator_value == 'yes'
+        run: |
+          echo self-approving
+          gh pr review --approve "$PR_URL"
+          gh pr merge --auto --squash "$PR_URL"