add health check job (#2266)

This should cut down on the number of comments we need to leave
regarding changelog updates etc.

.github/workflows/health.yaml

@@ -0,0 +1,10 @@
+name: Health
+on:
+  pull_request:
+    branches: [ main, master ]
+    types: [opened, synchronize, reopened, labeled, unlabeled]
+jobs:
+  health:
+    uses: dart-lang/ecosystem/.github/workflows/health.yaml@main
+    permissions:
+      pull-requests: write

.github/workflows/post_summaries.yaml

@@ -0,0 +1,18 @@
+name: Comment on the pull request
+
+on:
+  # Trigger this workflow after the Health workflow completes. This workflow
+  # will have permissions to do things like create comments on the PR, even if
+  # the original workflow couldn't.
+  workflow_run:
+    workflows:
+      - Health
+      - Publish
+    types:
+      - completed
+
+jobs:
+  upload:
+    uses: dart-lang/ecosystem/.github/workflows/post_summaries.yaml@main
+    permissions:
+      pull-requests: write