Support System.Text.Json serialization

[andreasohlund]

While there is a community system.text.json serializer we should consider supporting it as part of our offering. Since it's now part of .NET we should also consider including it as part of the core NServiceBus package. We also have the option of taking ownership of the community package if that makes things easier.

Related to https://github.com/Particular/NServiceBus/issues/6227