ci: add fast-forward PR merge workflow (#67)

.github/workflows/ff-merge.yml

@@ -0,0 +1,33 @@
+name: Fast-forward merge
+
+# This workflow helps to work around the bug in the **Rebase and Merge** pull requests strategy.
+# The bug leads to unsigned commits and prevents using the strategy
+# without IOG policy violation (all commits must be signed by PGP signature).
+#
+# This workflow helps to merge multiple commits from PR to main branch of the repository
+# without loosing of PGP signature.
+#
+# Related GitHub discussions: 
+# https://github.com/community/community/discussions/10410
+# https://github.com/orgs/community/discussions/5524
+
+on:
+  issue_comment:
+    types: [created]
+
+jobs:
+  fast_forward_job:
+    name: Fast Forward Merge
+    runs-on: ubuntu-latest
+    if: |
+      github.event.issue.pull_request != '' &&
+      contains(github.event.comment.body, '/fast-forward')
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v3
+      - name: Fast Forward Merge
+        uses: endre-spotlab/fast-forward-js-action@2.1
+        with:
+          GITHUB_TOKEN: ${{ secrets.ATALA_GITHUB_TOKEN }}
+          success_message: 'Success! Fast forwarded ***target_base*** to ***source_head***! ```git checkout target_base && git merge source_head --ff-only``` '
+          failure_message: 'Failed! Cannot do fast forward!'