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

Description

This issue is to implement Personal Access Token authentication with user_access keyword. This is useful to support agent access from the local terminal. See Design: Provide a user-based kubeconfig using t... (#331431) for more information.

It's also possible to use Project access tokens as it's basically service account + PAT.

Since glab uses PAT for authentication, we can easily generate KUBECONFIG for Design: Provide a user-based kubeconfig using t... (#331431).

A few notes:

• We've already implemented Browser cookie on GitLab frontend.

Implementation Guide

See full comment here: #409028 (comment 1487010922)

• Implement new k8s_proxy scope for PATs
  • *Awaiting input from @dblessing, see #409028 (comment 1488081442) *
• We need to extend the internal Kubernetes API endpoint at /kubernetes/authorize_proxy_user with a new access_type that is access_token. The current implementation always expects the access_type to be session_cookie.
• The only difference to the current implementation is how to get an authorized user from the provided access token.
  • For finding the user we should be able to use something like find_user_from_access_token
  • We also want to make sure that the provided token ONLY has the new k8s_proxy scope
  • The actual authorization for the agent is done in the Agent finder.

KAS Implementation Issue is here: Support Personal Access Token (PAT) authenticat... (gitlab-org/cluster-integration/gitlab-agent#439 - closed)