C#: support nullable contexts

[stephtr]

Starting with version 8.0 C# supports nullable reference types, which quicktype currently doesn't.

[mixa3607]

Maybe this help https://github.com/mixa3607/quicktype

[mindplay-dk]

@mixa3607 this looks good - you have support for partial classes in there as well. Why don't you submit a PR?

[Flern]

Looks like this is a bit old at this point... I made some updates to add .C# 8 version flag that triggers all types to be marked nullable here: #2694