Add Javascript proxy support

[amaitland]

I had this idea a while ago to use a Proxy in javascript to intercept method calls and dispatch them to .Net using Promises to fulfil the method calls. This is a basic prototype of that idea, should be something that can be made to work on all supported platforms.

• Add support for creating a javascript proxy that will intercept method calls and dispatch them to .Net (returning a promise).
• Replace JSInvokeTarget with HybridWebViewObjectHost that supports multiple objects (using a string key)
• Method invocation in javascript now returns a Promise that can be awaited to obtain the result.
• Methods in .Net can now return a Task that is awaited and the result sent back to javascript.
• OnMessageReceived was made protected as shouldn't be called directly by user

// .Net
// Add an object called 'host' that can be called from JavaScript
myHybridWebView.ObjectHost.AddObject("host", new MyJSInvokeTarget(this));

// JavaScript
// Directly call a .Net method called CallMeFromScript
HybridWebView.SendInvokeMessageToDotNet("host", "CallMeFromScript", ["msg from js", 987]);
// Create a proxy
const myDotNetObjectProxy = HybridWebView.CreateProxy("host");
// Call the method, await the result (promise) to get the returned value.
const result = await myDotNetObjectProxy.CallMeFromScriptReturn("Hello", 42);

Still lots of things I'd like to see added/supported

• ParamArray support
• Javascript -> Net method name conversion (should the conversion be done in javascript or .Net?)
• Javascript method names should probably be changed to start with lowercase to match javascript convention
• Test on all supported platforms (I've only tested on Windows/Android Emulator).
• Review method/event names and properly document public API.
• Improved error handling
• Review Threading, should methods be invoked on Worker Thread?
• Unit tests

[amaitland]

Android should now work (tested only in the emulator).

[saber-wang]

Android 9.0 execution failed(Chrome69)

[amaitland]

Android 9.0 execution failed(Chrome69)

That's expected, the browser is too old for the JavaScript in it's current form. In theory a minimum of version Chromium 74 would be required currently.

[amaitland]

I've changed the syntax slightly to lower the minimum required javascript version.