Extract GitLab-Shell JWT token verification into a class

What does this MR do and why?

Contributes to #456116 (closed)

Problem

GitLab-Shell JTW token extraction logic is defined in api/helpers.rb. That prevents us from reusing this code in other places.

Later it will be used here

Solution

Similar to other JWT verifications (like lib/gitlab/kas.rb), I extracted the common logic into Git::Shell#verify_api_request method.