Support for System.Text.Json.JsonDocument

[oliverjanik]

AspNetCore 3.0 dropped JSON.NET out of the box and replaced it with much more lightweight JsonDocument and JsonSerialiser

It seems this will be the official way to handle JSON for .NET going forward. It's still a bit rough, but I think it's worth investigating for this project.

As as example Npgsql has included direct JsonDocument support for the Postgres jsonb column out of the box recently.