Add Cell Manager

What does this merge request do and why?

This MR introduces the ability for developers to manage multiple local cell instances in their existing GDK instance.

Cells are stored in the gitlab-cells/ directory and are fully functional GDK instances. They are managed by the main GDK instance. We use local clones to bootstrap the

Ports are determined in the cells in GDK::CellsPortManager. This allows the developer to overwrite specific ports but also prevents port clashes (such as #1893 (closed)) by writing the cells.port_offset automatically from the main GDK instance to cell instances.

Based on !3685 (closed)

How to set up and validate locally

Check out the branch and follow the instructions in doc/howto/cells.md. Use GDK_SELF_UPDATE=0 gdk update instead of gdk update to avoid losing cells functionality.

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.