Added Livemode and Reason properties to ThinEvent

[jar-stripe]

Why?

The thin event payload has a boolean property called livemode that we need represented in the ThinEvent C# class. This PR adds this property and adds tests to verify it works correctly.

What?

• add Livemode and Reason properties to ThinEvent.cs
• add livemode json property to V2/EventTest.cs test data
• update tests to verify livemode is parsed
• added test for livemode: false
• added test for reason

See also

http://go/j/DEVSDK-2217

Changelog

• Update the class for ThinEvent to include Livemode and Reason