Add properties for Manage LIFF APIs

[hinoguma]

Need adding properties for liff.
Now some props (description, features... ) do not exist.
https://developers.line.biz/en/reference/liff-server/#add-liff-app

line-bot-sdk-go/linebot/liff.go

Lines 35 to 50 in 6948d5d

 // LIFFApp type 
 type LIFFApp struct { 
 	LIFFID string `json:"liffId"` 
 	View   View   `json:"view"` 
 } 
  
 // ViewRequest type 
 type ViewRequest struct { 
 	View View `json:"view"` 
 } 
  
 // View type 
 type View struct { 
 	Type LIFFViewType `json:"type"` 
 	URL  string       `json:"url"` 
 }