feat(app-check): Support Firebase AppCheck (#2940)

* Adding `@angular/fire/app-check`
  * Right now AppCheck doesn't "allow" you to set the debug token global right before initialization as it's inspected as a side-effect. I devised a hack that wins the race. Downside here is that Angular hasn't been initialized yet, so in order to tell if Angular is in devmode I'm looking up global variables, which are subject to behavior changes. Request has been submitted to the JS SDK team..
* Flushing out the samples a bit more, added appcheck
* Changing the tsconfig path resolution so it doesn't have to be exhaustive

Author: jamesdaniels

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@angular/fire",
-  "version": "7.0.4",
+  "version": "7.1.0",
   "description": "The official Angular library for Firebase.",
   "private": true,
   "scripts": {