Generic field.html.twig template for a custom Entity
Add your own theme suggestion:
mymodule.module
/**
 * Implements hook_theme_suggestions_field_alter().
 */
function mymodule_theme_suggestions_field_alter(array &amp;$suggestions, array $variables) {
  $element = $variables['element'];
  // place the least specific suggestion at the beginning of the array  
  array_unshift($suggestions, 'field__' . $element['#entity_type']);

}