(CAT-2056) - Add puppet-blacksmith to config_defaults

as we removed the build rake tasks from puppetlabs_spec_helper v8, we
now pull them into spec_helper through a soft dependency on
puppet-blacksmith. As such, we should add the puppet-blacksmith gem so
that users can still avail of rake tasks like module:build.

config_defaults.yml

@@ -558,6 +558,8 @@ Gemfile:
         version: '~> 4.0'
       - gem: 'puppetlabs_spec_helper'
         version: '~> 7.0'
+      - gem: 'puppet-blacksmith'
+        version: '~> 7.0'
     ':system_tests':
       - gem: 'puppet_litmus'
         version: