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

## 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
}
}
}
```