Add support for STJ-native C# code gen

[schnerring]

I took another crack at adding the missing System.Text.Json polymorphic serialization features to NSwag's C# client generator (previous PR: https://github.com/RicoSuter/NJsonSchema/pull/1595).

As per your suggestion I added a C# client generator setting named JsonPolymorphicSerializationStyle that defaults to NSwag (old behavior). Setting it to SystemTextJson will opt into the new behavior:

• Generate JsonPolymorphic and JsonDerivedType attributes
• Don't generate JsonInheritanceConverter and JsonInheritanceAttribute classes

I also added a corresponding (WIP) PR in the NSwag repo that might be missing a couple of things: https://github.com/RicoSuter/NSwag/pull/4782/files

[brrusino]

@schnerring @RicoSuter What's the latest status of this PR?

[schnerring]

@RicoSuter Anything else? It's good to go from my side.