Add support for Dictionary serialization when StronglyId is a key in System.Text.Json

[huberttrueselftrueme]

To add this support JsonConverter need to have override 2 methods.

public override Id ReadAsPropertyName(...)
public override void WriteAsPropertyName(...)

Now it is impossible to model like.

class MyFoo { prop Dictionary<StronglyId, ...> MyProperty {get; set;} }

[lucasteles]

looks reasonable!

Do you mind doing a PR on this?