Add lefthook to gitlab-qa

What does this MR do and why?

• Adds lefthook support to gitlab-qa
• Adds a lefthook rule to run rubocop in pre-push rules

NB - These configurations were taken directly from gitlab-org/gitlab - lefthook.yml

How to set up and validate locally

1. Install and setup lefthook

    ➜ bundle install  
    
    ➜ bundle exec lefthook install  
    SYNCING  
    SERVED HOOKS: pre-push  
    
    ➜ bundle exec lefthook run pre-push    
    Lefthook v1.2.6  
    RUNNING HOOK: pre-push  
    rubocop: (SKIP. NO FILES FOR INSPECTION)  
                     
    SUMMARY: (SKIP EMPTY) 

2. Verify git hook exists at .git/hooks/pre-push

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.