feat: add actor to system variables

[ainouzgali]

What change does this PR introduce?

Add actor to system variables, allows the use of the actor's subscriber data in editor + allows creation and update of the subscriber.

I still need to add tests.

Why was this change needed?

Other information (Screenshots)

[linear]

NV-2864 actor variables are not working in step editors like {{actor.firstName}}

Reproduction steps:-

1. create a workflow with in-app step

2. write this content in in-app editor

   {{subscriber.firstName}}, {{actor.firstName}} has mentioned you in {{page}}
   

3. Trigger the workflow as per below sample code

   await novu.subscribers.trigger("workflowIdentifier", {
     to: "subscriberId",
     payload: {
       "page": "Weekly Update page"
     },
     actor: "actorId"
   });
   

4. As per step 2 this should be content

   subscriberFirstName, actorFirstName has mentioned you in Weekly Update page
   

5. But actor first name will not be present. as actor variables are not accessible in editors

[scopsy]

🎉