Use subform mechanism for entity reference selection handler validation

Problem/Motivation

(Marking this as bug because there is no way to correctly implement SelectionInterface::validateConfigurationForm() due to this.)

Entity reference handlers can implement validation for their configuration via SelectionInterface::validateConfigurationForm(). However, when this is called by EntityReferenceItem::fieldSettingsFormValidate() the subform mechanism is not employed so that selection handlers get the entire form and form state without any way to know which part of the form they are responsible for.

Steps to reproduce

Proposed resolution

Employ the subform state mechanism to pass the plugin form and a SubFormState to the plugins.

Remaining tasks

User interface changes

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Introduced terminology

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API changes

Data model changes

Release notes snippet