code: func GetStringPolicy(d *schema.ResourceData, key string) *types.StringPolicy {
  	v, e := d.GetOkExists(key)
  	if e {
  		return StringPolicy(v.(string))
  	}
  	return nil
  }
docs: // GetStringPolicy reads a ResourceData and returns an appropriate StringPolicy
  // for the state of the definition. nil is returned if it does not exist.