Deprecate BundleFieldDefinition and BundlePluginInterface

I was leveraging Entity Traits and used the new BundleFieldDefinition class in Entity API; however, Commerce explicitly was pointing to its (effectively deprecated, identical child class) own implementation in the trait manager, which threw a PHP type mismatch error. This PR deprecates the class per the Drupal coding standard, updates the various references in Commerce core, and thus allows/encourages third parties to use the class from Entity module.