Add trash / soft deletion for ECK entities

[jensschuppe]

There should be means for "soft-deleting" ECK entities (a "trash bin" as exists for CiviCRM contact entities).

Reference:

• \Civi\Api4\Generic\Traits\SoftDeleteEntity
• \CRM_Contact_BAO_Contact::deleteContact()
• \CRM_Contact_BAO_Contact::contactTrash()

[colemanw]

FYI this issue made me realize that SoftDeleteEntity was misnamed so I renamed it.