Show config diff after `gdk reconfigure`

What does this merge request do and why?

After we've started to run gdk reconfigure in parallel we lost the ability to show changed configurations.

This commit runs diff-config before reconfigure and outputs any diffs after task has finished.

Refs !4290 (comment 2257846421)

Note this MR partially reverts !4125 (diffs, comment 2262799826) to buffer output again.

How to set up and validate locally

1. gdk config set openbao.enabled true
2. Ensure you have diffs: gdk diff-config
3. gdk reconfigure shows the diff

Before	After

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.