Graph `Activity` type member `ActivityTemplate` must describe the member `variables` detailed

[AlexAndBear]

Describe the bug

To make javascript-based clients use the type Activity we need to define the member variables in type ActivityTemplate properly, which is currently just type object.

This might be a helpful example:

type Activity = GraphActivity & {
  template: {
    variables?: {
      space?: SpaceResource
      resource?: Resource
      user?: User
    }
  }
}