Problem/Motivation
\Drupal\system\Entity\Action::execute() does not exist on any interface. This forces integrating code to typehint on the actual class instead of any interface.
Proposed resolution
Add execute() to \Drupal\system\ActionConfigEntityInterface.
API changes
Because there is a 1-1 relationship between \Drupal\system\Entity\Action and \Drupal\system\ActionConfigEntityInterface this is permissible in 8.x due to the Interfaces within non-experimental, non-test modules not tagged with either @api or @internal section of https://www.drupal.org/core/d8-bc-policy
However, we should add a change notice.