Problem/Motivation
\Drupal\Core\Field\FieldItemList::defaultValueWidget() changes the required and description values of the field definition while rendering the default values form, but the problem is that these values aren't restored after rendering, which means that any code coming after this receives wrong description and required values when reading from the field definition. This was discovered in a contrib module reading the required value: #3460530: Required checkbox has the wrong value when using the default (Core) Required API plugin.
Steps to reproduce
Implement hook_form_field_config_edit_form_alter and read the required value. It will always be false.
Proposed resolution
clone the field definition object before modifying.
