Fixes session routing in GDK

What does this merge request do and why?

This MR is follow-up from #2229 (closed), where we wanted to configure the GITLAB_RULES_CONFIG variable from the gdk.yml file.

How to set up and validate locally

1. Checkout the branch 2229-fixes-session-routing-in-gdk
2. Add gitlab_rules_config value to the gitlab_http_router section of the gdk.yml file.
3. Run gdk reconfigure
4. Verify that new value has been added to the wrangler.toml file 🎉

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.

Closes #2229 (closed)