Add a workflow to make sure release notes are updated

When a PR is changing files in the `proto` or `py` directories, this
workflow will fail if the release notes weren't updated in the PR too,
unless the label `cmd:skip-release-notes` is set in the PR, which
signals that this PR doesn't need a release notes update.

Signed-off-by: Leandro Lucarella <[email protected]>

Author: llucax

.github/workflows/release-notes-check.yml

@@ -0,0 +1,28 @@
+name: Release Notes Check
+
+on:
+  merge_group:
+  pull_request:
+    types:
+      # On by default if you specify no types.
+      - "opened"
+      - "reopened"
+      - "synchronize"
+      # For `skip-label` only.
+      - "labeled"
+      - "unlabeled"
+
+
+jobs:
+  check-release-notes:
+    name: Check release notes are updated
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check for a release notes update
+        if: github.event_name == 'pull_request'
+        uses: brettcannon/check-for-changed-files@4170644959a21843b31f1181f2a1761d65ef4791 # v1.2.0
+        with:
+          file-pattern: "RELEASE_NOTES.md"
+          prereq-pattern: "src/**"
+          skip-label: "cmd:skip-release-notes"
+          failure-message: "Missing a release notes update. Please add one or apply the ${skip-label} label to the pull request"