Add Flutter Mobile Demo / Maybe desktop & web

[fundthmcalculus]

Add flutter mobile demo application with the following support:

• Android (initial test works)
• iOS (Still needs testing)
• Web? (https://gpalma.pt/blog/flutter-web-web-assembly/) - hard maybe this can use the existing WASM binaries, but worth a look at some point?
• Windows (already tested)
• Linux (needs tested)
• MacOS (needs tested)

[tomquas]

@fundthmcalculus thanks for the great work! i was able to integrate libokapi with flutter on android, but failed to make it work on ios yet.

while my flutter tests work on a laptop with the provided macos libs, whenever i try to to run on a ios device, DynamicLibrary.process() fails – it seems libokapi.a is being optimized away from the build, no matter where i put it in the xcode config.

some recommended to call functions exposed by the library in Runner-Bridging-Header, similar to what GeneratedPluginRegistrant.m does, to prevent xcode from being overpedantic.

can you help?