chore: add workflow to sync public and private repositories.

This commit adds a workflow that force pushes the public repository state
to the private repository on pushes to main.

.github/workflows/sync_on_push.yaml

@@ -0,0 +1,31 @@
+# Sync repos
+name: Sync repos
+
+on:
+  push:
+    branches:
+      - 'main'
+  workflow_dispatch:
+
+jobs:
+  sync-repo:
+    if: ${{ github.repository == 'zama-ai/concrete-ml' }}
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout repo
+        uses: actions/checkout@c85c95e3d7251135ab7dc9ce3241c5835cc595a9
+        with:
+          fetch-depth: 0
+      - name: Save repo
+        uses: actions/upload-artifact@0b7f8abb1508181956e8e162db84b466c27e18ce
+        with:
+          name: repo-archive
+          path: '.'
+      - name: git-sync
+        uses: wei/git-sync@55c6b63b4f21607da0e9877ca9b4d11a29fc6d83
+        with:
+          source_repo: "zama-ai/concrete-ml"
+          source_branch: "main"
+          destination_repo: "https://${{ secrets.BOT_USERNAME }}:${{ secrets.CONCRETE_ACTIONS_TOKEN }}@github.com/${{ secrets.SYNC_DEST_REPO }}"
+          destination_branch: "main"
+