Support PAT configuration for gitlab-shell

What does this merge request do and why?

Support PAT configuration for gitlab-shell

Contributes to gitlab#460290 (closed)

New configuration options were added to gitlab-shell: gitlab-shell!1141 (merged).

This MR adds new GDK settings for Personal access tokens configuration:

pat:
  enabled: true # it's enabled by default, the new configuration option allows to disable it
  allowed_scopes: [] # when scopes are empty (default state), we skip the scope validation. See: https://gitlab.com/gitlab-org/gitlab-shell/-/merge_requests/1053/diffs#20765132b773f85253f8965a7531f37b6b598971_63_67

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.