[C#] Codegen for Unity

Description

Even though there's two generators for C# (csharp and csharp-dotnet2) both relying on RestSharp, and also both can be adapted to work with Unity, the game engine. But here's my motivation for a separate generator for Unity:

• Using UnityWebRequest which is the fastest and has the lowest memory footprint across all platforms Unity supports, for any build settings. In addition, it will result in a smaller build sizes after removing (also, it automatically handles special cases and platforms, such as WebGL, where the .NET networking
  classes (ie, everything in the System.Net namespace, particularly System.Net.Sockets) are non-functional in WebGL.)

• Using a version of Json.NET that is adapted for Unity (for both Mono and IL2CPP platforms)

• A generated code that automatically handles Scripting restrictions (for example, certain types will be stripped out in a IL2CPP build, unless they are used in the code somehwhere, or else json deserialization could fail. The generated code can be done to take this into account)

• Handing async operations Unity style (using Coroutines and/or async/await methods, depending on current player and build settings, using platform dependent compilation defines)

• Adding some sugar (assembly definition files, scriptable objects to store configurations, custom inspectors to make calls to codegen and regenerate the code - see picture below as an example - , and so on)
  

• Exposing only relevant options to Unity in the generator

• Removing irrelevant files (e.g. no need for build scripts or C# solution files)

• Tests organised in a way to run well in Unity's test runner

Please let me know what do you think.

Review the existing csharp and csharp-dotnet2 generators first, then define the separate Unity generator around UnityWebRequest, Unity-compatible Json.NET, Unity-style async handling, and platform restrictions. Done would mean the Unity-specific generator covers the requested options, Unity-oriented files and configuration, and tests that run in Unity's test runner.