Add a roll-up semi-optional status action (#26939)

It check status on a pr, and when some set are successful it makes itself successful.

If any that it's watching fail, it fails.

If the force-merge label is applied, it succeeds.

With the plan: we make EasyCLA and probably Sanity, and this one required statuses for merging (i.e. anything that should not be bypassed still gets verified by GitHub)

We make every other status that we believe in (so everything but the Google3 presubmits because they're fresh?) something that this script verifies.

That lets us do automerge (all things are passing)

And if all things are not passing we can use human judgement and apply the force-merge label to bypass these checks... But importantly not the CLA check

.github/workflows/happy-pancakes.yml

@@ -0,0 +1,74 @@
+name: Happy Pancakes
+on:
+  pull_request:
+    types: [opened, edited, labeled, unlabeled, synchronize, ready_for_review, auto_merge_enabled, auto_merge_disabled]
+jobs:
+  check_status:
+    name: Check Required Status or force-merge label
+    runs-on: ubuntu-latest
+    steps:
+      # Cancel current runs if they're still running
+      # (saves processing on fast pushes)
+      - name: Cancel Previous Runs
+        uses: styfle/cancel-workflow-action@0.9.1
+        with:
+          access_token: ${{ github.token }}
+      # Wait until our required statuses are ready or failed
+      # Unless force-merge is present, and then just be good.
+      - name: Check Status
+        uses: actions/github-script@v4
+        with:
+          script: |
+            function sleep(ms) {
+              return new Promise(resolve => setTimeout(resolve, ms));
+            }
+            let pull = await github.pulls.get({
+              owner: "grpc",
+              repo: "grpc",
+              pull_number: ${{ github.event.pull_request.number }}
+            });
+            for (var i = 0; i < pull.data.labels.length; i++) {
+              let label = pull.data.labels[i];
+              if (label.name == "force-merge") {
+                console.log("Saw force-merge label");
+                return;
+              }
+            }
+            while (true) {
+              var need = new Set();
+              need.add("Bazel Basic build for C/C++");
+              need.add("Portability Tests Linux [Build Only] (internal CI)");
+              for (var page=1;; page++) {
+                console.log("***** page " + page);
+                let statuses = await github.repos.listCommitStatusesForRef({
+                  owner: "grpc",
+                  repo: "grpc",
+                  ref: "${{ github.event.pull_request.head.sha }}",
+                  per_page: 100,
+                  page: page
+                });
+                for (var i = 0; i < statuses.data.length; i++) {
+                  let status = statuses.data[i];
+                  console.log(status.context, status.state, need);
+                  if (need.has(status.context)) {
+                    if (status.state == "success") {
+                      need.delete(status.context);
+                    } else if (status.state == "pending") {
+                      // do nothing
+                    } else {
+                      core.setFailed("Required status failed: " + status.context);
+                      return;
+                    }
+                  }
+                }
+                if (statuses.data.length < 100) {
+                  break;
+                }
+              }
+              if (need.size == 0) {
+                return;
+              }
+              // Sleep 15 minutes between polls
+              await sleep(15 * 60 * 1000);
+            }
+