stax should support stack policies

[talentedmrjones]

CloudFormation stacks can have policies that limit what resources can be updated or deleted. A stack policy is set by its own operation cloudformation:SetStackPolicy. I propose the following solution:

Stack object supports a Policy field like so:

Stack: [string]: {
  Policy?: {
    Body?: {
      Statement:[...{
          Effect: "Allow" | "Deny"
          Action: string
          Principal: string
          Resource : string
        }
      ]
    }
    Url?: string // regex would be better
  }
}

A new command policy <set | get | delete> would allow direct management of the policy. The deploy command could call policy set after successful deployment but would require a flag or setting in config.stax.cue