code: func (c *Client) AddonList(appIdentity string, lr *ListRange) ([]Addon, error) {
  	req, err := c.NewRequest("GET", "/apps/"+appIdentity+"/addons", nil, nil)
  	if err != nil {
  		return nil, err
  	}
  
  	if lr != nil {
  		lr.SetHeader(req)
  	}
  
  	var addonsRes []Addon
  	return addonsRes, c.DoReq(req, &addonsRes)
  }
docs: // List existing add-ons.
  //
  // appIdentity is the unique identifier of the Addon's App. lr is an optional
  // ListRange that sets the Range options for the paginated list of results.