Expose optional autoscaling fields in emr notebook

[santina]

(Haven't tested yet)

[santina]

Related to #43

[adrianisk]

I'm guessing some of these autoscaling properties are grouped together (you have to have a role if you have a policy, etc). Might be better to make a single variable who's type is an object with all of these properties as strings. That way it's very clear to end-users that you need to provide all of them if you want autoscaling, and the default for the variable can just be null to indicate no autoscaling.

Something like

var "autoscaling_config" {
type = object ({
    idle_timeout = number
    autoscaling_role = string
    autoscaling_policy = string
    })
}
default = null
description = "See https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/emr_cluster"