Add support for non-string discriminators using System.Text.Json

[david-brink-talogy]

System.Text.Json supports non-string discriminators using JsonDerivedType. This PR stringifies the discriminator value.

Fixes #1723

From https://learn.microsoft.com/en-us/dotnet/standard/serialization/system-text-json/polymorphism?pivots=dotnet-8-0#mix-and-match-type-discriminator-formats

[JsonDerivedType(typeof(ThreeDimensionalPoint), typeDiscriminator: 3)]
[JsonDerivedType(typeof(FourDimensionalPoint), typeDiscriminator: "4d")]
public class BasePoint
{
    public int X { get; set; }
    public int Y { get; set; }
}

public class ThreeDimensionalPoint : BasePoint
{
    public int Z { get; set; }
}

public sealed class FourDimensionalPoint : ThreeDimensionalPoint
{
    public int W { get; set; }
}