Compile a cross-platform websockets library

[GabrielePicco]

Describe the solution you'd like
For SMS, Account/Transaction subscription, fixing the Streaming RPC client we need a websocket that works cross-platform.

• Built a single assembly (e.g.: websocket-sharp-unity.dll) that contains an interface for use and hide cross-platform complexity.

Describe alternatives you've considered
Combination of websocket-sharp for NoWebGL and NativeWebSocket for WebGL seems to work on all the platform.

See #32 for additional context.