Add Dependabot auto-merge workflow

Use commit hash for dependabot-auto-approve action for better
security and reproducibility.

Signed-off-by: Mathias L. Baumann <mathias.baumann@frequenz.com>

Author: Marenz

.github/workflows/auto-dependabot.yaml

@@ -0,0 +1,19 @@
+name: Auto-merge Dependabot PRs
+
+on:
+  pull_request:
+
+permissions:
+  contents: write
+  pull-requests: write
+
+jobs:
+  auto-merge:
+    if: github.actor == 'dependabot[bot]'
+    runs-on: ubuntu-latest
+    steps:
+      - name: Auto-merge Dependabot PR
+        uses: frequenz-floss/dependabot-auto-approve@3cad5f42e79296505473325ac6636be897c8b8a1 # v1.3.2
+        with:
+          github-token: ${{ secrets.GITHUB_TOKEN }}
+          merge-method: 'merge'