[c#] csharp and csharp-dotnet2 needs new codegen

[Fildrance]

Description

'csharp' and 'csharp-dotnet2' codegens are obsolete to say the least. While i do like very much what other parts of swagger-codegen-cli generate (as its default conventions and infrastructure seems very robust) generators needs updates / replacement to keep up with trending http-clients and language features.

• Usage of M$-provided HttpClient is a safe pick for client implementation, no forced dependencies and optimal performance for pretty much most of stable .NET Core versions
• System.Text.Json as serializer works pretty much fine on most cases when api works in a way that swagger can describe it.
• usage of mono for building is not overly useful as dotnet is crossplatform without usage of mono right now

Swagger-codegen version

looking at 3.0.54

Suggest a fix/enhancement

Making some kind of clone for csharp-dotnet2 and refining code that generates clients and additional files would be nice.

[Fildrance]

I can do it but i'd like to know

1. How long such 'non-essential' feature will be waiting for PR to be merged? I know by amount of issues about csharp lang on repo that this is not really most demanded by users, or at least it looks so by a first glance.
2. How should new generator be called? having csharp-dotnet2 that is tied to mono is aready huge pain. Calling it csharp-dotnet3 will be not even fun. Main target is create something compatible with netstandard2.0 but with options that will support c# 10 language features. Changing whole csharp-dotnet2 doesnt feel right as there wont be any backwards compatibility, and some people with ancient projects can be sad about it.