ci: Add workflow to prevent merging PRs with 'dont-merge' label

Sometimes, it is useful to have an option to prevent maintainers from
merging a Pull Request when reviews are in and all checks are green but
merging should be deferred for whatever reason. This commit adds a
workflow that fails if a Pull Request has a label containing the
substring 'dont-merge' [0].

This mechanism is inspired by what Cilium does with maintainer's little
helper [1] and the implementation is based on this blog post from Jesse
Squires and Ben Asher [2].

[0] https://docs.github.com/en/actions/writing-workflows/choosing-what-your-workflow-does/evaluate-expressions-in-workflows-and-actions#contains
[1] https://github.com/cilium/github-actions/blob/d6fb4a25efefa0f830dce582af2510eb7eed6397/README.md?plain=1#L55
[2] https://www.jessesquires.com/blog/2021/08/24/useful-label-based-github-actions-workflows/#do-not-merge

Signed-off-by: Quentin Monnet <[email protected]>

Author: qmonnet

.github/workflows/mergeability.yml

@@ -0,0 +1,16 @@
+name: Mergeability
+
+on:
+  pull_request:
+    types: [synchronize, opened, reopened, labeled, unlabeled]
+
+jobs:
+  check-labels:
+    runs-on: ubuntu-latest
+    name: Check mergeability based on labels
+    steps:
+      - if: ${{ contains(github.event.*.labels.*.name, 'dont-merge') }}
+        name: Fail test due to 'dont-merge' label presence
+        run: exit 1
+      - name: Allow merging
+        run: exit 0