Add an option to use System.Text.Json instead of NewtonsoftJson

[j-mok]

I'm wondering if there is currently any particular reason the platform uses NewtonsoftJson instead of the now-default System.Text.Json for ASP.NET MVC? Are there any particular customizations that would be difficult to pull of with System.Text.Json. PolymorphJsonContractResolver maybe?

I think it would be beneficial to allow VC instances to optionally use the default serializer. This has a number of advantages:

• System.Text.Json tends to be faster
• C# 11's required keyword is supported by System.Text.Json and helps in model validation
• There are some promising source-generation techniques available with System.Text.Json that could help improve cold-start times in the AOT fashion