No POS Facade

Merge pull request #5 from IndieSquare/webGLsupport

BTCpay removed POS facade

first WebGL build support

To support WebGL build, we have done the following

1.remove Threading/Task/async-await as much as possible but use Unity CoRoutine
2.remove .net WebSocket class and introduce jslib WebSocket for WebGL and websocket-csharp for others
introduce "Json for Unity" package from Asset Store
3.Stripping prevention with link.xml
4.Remove heavy dependency on BouncyCastle and use NBitcoin for Key generation

Still lots of todo, like clean-up (debug logging) and dll for the ease of use by users.

Unity Btcpay sdk

1.Remove dependency on the websocket-sharp and utilize the csharp's native websocket API
2.Use async-await instead of Coroutine
3. Softly force Facade "pos"