JsonSerializer could have an option to include serialization of Fields #3920

Have this exciting discussion about how to bend an old application to support structured logging:

https://stackoverflow.com/questions/61281449/set-valueformatter-for-specific-nlog-instance

There are some places where NLog could be more helpful for this scenario:

• When replacing the JsonSerializer with a custom one, then one doesn't actually get to know the configured MaxRecursionLimit on the JsonLayout. I think it would make sense that the JsonLayout skips calling the JsonSerializer for "unknown" objects when MaxRecursionLimit=0. But instead just performs a ToString-operation (Unknown objects are those that doesn't implement IFormattable-interface)
• The NLog JsonSerializer could have an option to include serialization of Fields, and not just Properties. Also noticed that DefaultJsonSerializer is a public class, but it should have been internal one. Maybe change this for NLog 5. Instead there should be a Setup-method that allows one to update the NLog JsonSerializeOptions for the default NLog JsonSerializer.