Implement the dictionary key (de)serialization for the System.Text.Json converter

[Varveyn]

Since dotnet/runtime#46520 has been implemented, the JsonConverter now has the ReadAsPropertyName and WriteAsPropertyName methods, which could be used to allow using strongly-typed IDs as dictionary keys during the (de)serialization. Would be great to have this behavior out-of-the-box.

[andrewlock]

Hi @Varveyn, I'm currently working on a big redesign of the library in this PR:

• Fundamental redesign to allow complete customisation #117

The main idea is to make the library much more maintainable while also giving people a mechanism to customise the generated IDs as much as they like.

I've included this fix as part of that redesign!