Enable BlazorAppScheme to be a configurable value

[trevjacobson]

Looking to be able to override the PhotinoWebViewManager.BlazorAppScheme value to be file (and in turn affecting PhotinoWebViewManager.AppBaseUri) which will enable file system access from the webview process, not limited to the wwwroot published folder for an offline desktop application.

[philippjbauer]

@trevjacobson Can you provide some more information on your use case? Usually, direct access from a browser engine is not permitted due to the sandbox they run in. Loading the index.html using the file:// scheme is not supported.

You can use C# to open and manipulate files and select files for this purpose using the typical JS APIs.

Using the file scheme as you (seem) to propose could also create issues with differences in file path structure across operating systems. E.g. Windows using something like C:\.... while UNIX type OSs are using /home/username/...

https://stackoverflow.com/a/12711835
https://stackoverflow.com/a/372333