Warn when PR may require regenerating cloud PG settings

These generated Postgres settings JSON files can get out of sync causing
the control plane to reject updated to an endpoint or project's Postgres
settings.

Signed-off-by: Tristan Partin <[email protected]>

.github/workflows/regenerate-pg-setting.yml

@@ -0,0 +1,41 @@
+name: Regenerate Postgres Settings
+
+on:
+  pull_request:
+    types:
+      - opened
+      - synchronize
+      - reopened
+    paths:
+      - pgxn/neon/**.c
+      - vendor/postgres-v*
+      - vendor/revisions.json
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.head_ref }}
+  cancel-in-progress: true
+
+permissions:
+  pull-requests: write
+
+jobs:
+  regenerate-pg-settings:
+    runs-on: ubuntu-22.04
+
+    steps:
+      - name: Add comment
+        uses: thollander/actions-comment-pull-request@v3
+        with:
+          comment-tag: ${{ github.job }}
+          pr-number: ${{ github.event.number }}
+          message: |
+            If this PR added a GUC in the Postgres fork or `neon` extension,
+            please regenerate the Postgres settings in the `cloud` repo:
+
+            ```
+            make NEON_WORKDIR=path/to/neon/checkout \
+              -C goapp/internal/shareddomain/postgres generate
+            ```
+
+            If you're an external contributor, a Neon employee will assist in
+            making sure this step is done.