Add ContainerFactoryPluginInterface to Drupal\Core\Field\WidgetBase and Drupal\Core\Field\FormatterBase

Problem/Motivation

After visiting @alexpott's session "Drupal 9 is coming: Getting your code ready
" (https://events.drupal.org/seattle2019/sessions/drupal-9-coming-getting-y...) in Seattle, I wanted to make the contrib select2 module more BC stable by preventing constructor overwrites.

I found out that I am not able to prevent constructor overwriting for the Select2EntityReferenceWidget (https://git.drupalcode.org/project/select2/blob/8.x-1.x/src/Plugin/Field...) because none of it's base classes is implementing the ContainerFactoryPluginInterface.
The base class chain of this class is Select2EntityReferenceWidget -> Select2Widget -> OptionsSelectWidget -> OptionsWidgetBase -> WidgetBase. That means if one of this base classes changes it's constructor, I will have a breakage in Select2EntityReferenceWidget.

Proposed resolution

Add ContainerFactoryPluginInterface to Drupal\Core\Field\WidgetBase and Drupal\Core\Field\FormatterBase and implement the ::create() method in them. The big advantage is that all child classes will be more BC stable.

Remaining tasks

Discuss
Implement
Test
Commit

User interface changes

n.a.

API changes

Add ContainerFactoryPluginInterface to Drupal\Core\Field\WidgetBase

Data model changes

n.a.

Release notes snippet

n.a.