Update Entity Timestamp if possible

Problem/Motivation

If the Entity used by an EntityResource implements `EntityChangedInterface` this is not updated when a patch method is used to update the entity via the rest API.

Steps to reproduce

Create an entity that implements EntityChangedInterface and a subclass of EntityResource which uses this Entity. Update the entity via the rest API, the updated timestamp is not modified.

Proposed resolution

This is a fairly simple enhancment

Before the entity is saved in the Patch method

if ($original_entity instanceof EntityChangedInterface) {
      $original_entity->setChangedTime(time());
    }

This is a core interface so it would be nice to see it supported.

Remaining tasks

User interface changes

None

API changes

None

Data model changes

None