Ensure initialization w/CertCredential has needed scopes (#58)

While using CertCredential, against a newly issued Firebase Admin SDK private key, I am seeing scope failures getting the Firebase environment configuration. Adding `https://www.googleapis.com/auth/cloudruntimeconfig` to the `AUTH_SCOPES` allows this to function as expected.
Further, we should allow access to all firebase w/admin-sdk credentials.

Author: jamesdaniels

src/credential.ts

@@ -44,7 +44,8 @@ const ONE_HOUR_IN_SECONDS = 60 * 60;
 const JWT_ALGORITHM = 'RS256';
 const AUTH_SCOPES = [
   'https://www.googleapis.com/auth/userinfo.email',
-  'https://www.googleapis.com/auth/firebase.database',
+  'https://www.googleapis.com/auth/cloudruntimeconfig',
+  'https://www.googleapis.com/auth/firebase',
 ];
 
 /**