GDK::Command: Add gdk config set subcommand

What does this Merge Request do and why?

This change adds the command gdk config set. It allows the user to write a value to gdk.yml with a simple command.

Usage:

gdk config set <name> <value>

Fixes: #939 (closed)

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 Issue to follow-up.
• Documentation added/updated, if needed.
• [-] gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.