Messaging API - August 2020 update

[tkgauri]

News: https://developers.line.biz/en/news/2020/08/04/messaging-api-update-august-2020/

• Unsend event has been added Support unsend event. #565
• Video viewing complete event has been added Video viewing complete event has been added #560 #570
• The audiences property is now optional instead of required for a certain endpoint
• Create audience for uploading user IDs (by file) Manage audience upload by file #573
• Add user IDs or Identifiers for Advertisers (IFAs) to an audience for uploading user IDs (by file Manage audience upload by file #573

[tokuhirom]

Video play complete event is #570

[tokuhirom]

The audiences property is now optional instead of required for a certain endpoint doesn't require any code change in this SDK.