Implements #7181: added initial support for rubocop

I've updated the Jenkins job and I think it's failing due to this:

<pre>
Warning: unrecognized cop Metrics/BlockNesting found in /var/lib/workspace/workspace/test_proxy_develop_pr_rubocop/.rubocop_todo.yml
Warning: unrecognized cop Metrics/BlockNesting found in /var/lib/workspace/workspace/test_proxy_develop_pr_rubocop/.rubocop.yml
</pre>