Draft: Add unused let

What does this MR do and why?

Adding a new rubocop rule that will pick up a let variable in rspec that is not used.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Add unused let variable to any rspec file and try to commit the change. You should see a similar message to the one above ☝

rubocop todo generated via bundle exec rake rubocop:todo:generate[Rspec/UnusedLet,Lint/Syntax]