Refactor Shellout and move inside GDK

What does this merge request do and why?

Ideally no code should live outside of the GDK module to avoid conflicts with other code being loaded (for example from other gems).

This refactors the Shellout code that lives outside of GDK module, and move it to be inside, and make some quality of life improvements:

1. Move Shellout to be inside GDK module
2. Refactor Shellout usages
3. Introduce ShelloutHelper for specs

How to set up and validate locally

There is no change in behavior. Test coverage should be enough.

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.