Add `gdk config list` command

What does this merge request do and why?

List all available config variables.

$ gdk config list
---
action_cable:
  worker_pool_size: 4
asdf:
  opt_out: false
cells:
  enabled: false
  postgresql_clusterwide:
    host: "/workspace/gitlab-development-kit/postgresql"
    port: 5432
charts_gitlab:
  auto_update: true
  enabled: false
...

How to set up and validate locally

Run gdk config list to see all available options.

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.

Add