[FZ Editor] Consider using JsonSerializer

[enricogior]

FZ engine and FZ editor use temp files to exchange data in json format.
Consider using JsonSerializer for the C# code and investigate what is available in C++ for automatic object serialization to json file.

JsonSerializer requires .NET Core 3.x, so this work-item depends on #776

[crutkas]

my feedback boils down to two items:

1. Feels odd we are writing directly versus serializing an object
2. regardless of implementation is single spot for serializing / deseralizing. (C++ vs C#)

[enricogior]

We should also consider what we can use on ARM, and in case simply drop the use of temp files towards a more robust IPC approach.

[crutkas]

I think this is part of .net core 3 upgrade as well as migrating to System.Text.Json.

[enricogior]

This has been done.