Add kill switch mechanism

This code implements the kill switch mechanism in the extension. This is
explained in //src/killSwitch/README.md and in the design doc:
https://docs.google.com/document/d/1O5YV6_WcxwrUyz-lwHOSTfZ3oyIFWj2EQee0VuKkhaA/edit.

Bug: twpowertools:64

Change-Id: Ia993c78035bba7038aafd53d156f20954217e86f
diff --git a/package.json b/package.json
index 33e929e..bd3086e 100644
--- a/package.json
+++ b/package.json
@@ -34,7 +34,11 @@
   },
   "private": true,
   "dependencies": {
+    "google-protobuf": "^3.18.0-rc.2",
+    "grpc-web": "^1.2.1",
     "idb": "^6.1.2",
-    "poll-until-promise": "^3.6.1"
+    "poll-until-promise": "^3.6.1",
+    "semver": "^7.3.5",
+    "sw-xhr": "^0.0.3"
   }
 }