Problem/Motivation
Some layout plugins choose to provide configuration forms.
Those are form_alter-able by custom code.
However, if a layout plugin does not provide its own form, custom code can't do anything about it without swapping out the plugin class itself.
Proposed resolution
This is what PluginWithFormsInterface is for!
Add that to LayoutDefault (the 99.9% use case) and it will allow custom code to alter the layout definition to provide a form for them, without messing with their plugin class.
Remaining tasks
Write tests
User interface changes
API changes
None. LayoutInterface is not changed here (ideally we'd be able to, but erring on the side of no API addition)
