Add asdf-install target for gitlab-pages

What does this merge request do and why?

Similar to !3454 (merged), this MR introduces gitlab-pages-asdf-install target.

Related to #1635 (closed)

How to set up and validate locally

1. git checkout 1635-asdf-install-in-gitlab-pages
2. gdk config set gitlab_pages.enabled true
3. gdk reconfigure

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Installing asdf tools from /src/gdk/gitlab-pages/.tool-versions
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golang 1.21.4 is already installed

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.