[AAE-4966] Extensible app config (#6914)

* merge app config from the extensions

* improved service injection in unit tests

* extra unit test

* fix content tests

* update code as per review

* fix lint

* fix lint

* update code and tests

* update schema

lib/core/testing/core.testing.module.ts

@@ -40,8 +40,8 @@ import { VersionCompatibilityService } from '../services/version-compatibility.s
         NoopAnimationsModule,
         RouterTestingModule,
         HttpClientModule,
-        TranslateModule,
-        CoreModule
+        TranslateModule.forRoot(),
+        CoreModule.forRoot()
     ],
     providers: [
         DatePipe,