Add `gitlab-rspec` gem

What does this MR do and why?

Describe in detail what your merge request does and why.

Add gitlab-rspec gem

This gem is supposed to contain all source code required to configure GitLab specific rspec environment.

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.