Make a generic layout builder or optional layout_plugin integration

Problem/Motivation

Core has experimental layout discovery and field layout plugins since 8.x-3: #2296423: Implement layout plugin type in core. Explore possibility to make GridStack a generic layout builder so that GridStack layouts may be consumed by DS, Panels, etc.

Proposed resolution

• Optionally integrate with layout_plugin.module.
• Alternatively postpone the intention for core integration at 8.x-3.

Remaining tasks

• Learn how to optionally integrate with layout_plugin.module, or directly with core 8.x-3.

User interface changes

None.

API changes

None.

Data model changes

None.