Inject `.gdkrc` into user's shell profile

What does this merge request do and why?

• Create rake task gdk:configure_shell to load .gdkrc from user's shell profile
• Add configure shell rake task to gdk reconfigure

Closes #2534

This should fix #2522 (closed) but I will test specifically for postgres 16 upgrade before closing that issue.

How to set up and validate locally

1. Check out branch
2. Check that shell profile does not load .gdkrc
3. Ensure GDK_ENV_LOADED is not set: unset GDK_ENV_LOADED
4. Run bundle exec rake gdk:configure_shell

Should see: "Configured shell to load .gdkrc" Check that shell profile loads .gdkrc

After restarting shell:

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 MR references an issue describing the change.
• 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.