config: Allow users to opt-out from installing system packages

What does this merge request do and why?

GDK will install system packages as needed. But some users might not want that. For them the new setting gdk.system_packages_opt_out can be set to true, and GDK will not attempt to install packages.

How to set up and validate locally

1. Run gdk config set gdk.system_packages_opt_out true
2. Run support/bootstrap-packages

No packages should be attempted to install.

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.