build: set up post approval changes commit action

This action will enforce that all pull requests receive an approval from
at least one googler on the final commit for the pull request. Historically,
we have allowed all post approval changes regardless of authorship. Moving
forward, with this change, we will only allow known googlers to perform
post approval changes.

When a post approval change occurs by a non-googler, the action will
automatically rerequest a review from the latest googler who provided
an approval on the change.

(cherry picked from commit b8fc4c7)

Author: alan-agius4

.github/workflows/dev-infra.yml

@@ -1,13 +1,13 @@
 name: DevInfra
 
-# Declare default permissions as read only.
-permissions:
-  contents: read
-
 on:
   pull_request_target:
     types: [opened, synchronize, reopened]
 
+# Declare default permissions as read only.
+permissions:
+  contents: read
+
 jobs:
   labels:
     runs-on: ubuntu-latest
@@ -16,3 +16,10 @@ jobs:
       - uses: angular/dev-infra/github-actions/commit-message-based-labels@2a04da0754b050fb17d6cfc4a4b4fa8b5575ea86
         with:
           angular-robot-key: ${{ secrets.ANGULAR_ROBOT_PRIVATE_KEY }}
+  post_approval_changes:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@2541b1294d2704b0964813337f33b291d3f8596b # tag=v3.0.2
+      - uses: angular/dev-infra/github-actions/post-approval-changes@2a04da0754b050fb17d6cfc4a4b4fa8b5575ea86
+        with:
+          angular-robot-key: ${{ secrets.ANGULAR_ROBOT_PRIVATE_KEY }}