Support Personal Access Token (PAT) authentication with `user_access`

This issue implements the KAS part of (Rails) Support Personal Access Token (PAT) aut... (gitlab-org/gitlab#409028 - closed)

Implementation Guide

• Read the token from the Authorization header and check if it starts with pat: otherwise it's the already existing job token authentication
• Implement a call to the Rails /internal/kubernetes/authorize_proxy_user API similar to the session cookie path.