Problem/Motivation
The text format configure form contains the forms of all of its filter plugins.
The block entity form contains the form of its block plugin.
The view entity contains the forms of its displays, rows, styles...
Condition plugins currently use FormInterface, but that is intended for a standalone form being passed to drupal_build_form().
Proposed resolution
Introduce a new interface similar to FormInterface, but without getFormID().
This should not be explicitly tied to plugins, but they will be the main use case for this.
Remaining tasks
Will there ever be a case where a form is to stand alone or be used as a sub form?
If so we'll need different method names, which we might want anyway.
Possibly consider a better name than SubFormInterface.
User interface changes
N/A
API changes
Addition