New: System.Text.Json Source Generator

[Zetanova]

In .net6 introduces a native System.Text.Json Source Generator
https://devblogs.microsoft.com/dotnet/try-the-new-system-text-json-source-generator/

This would be a great source for performance increase.

The only thing that I could not figure out,
how to use it, without specifying the correct JsonSerializerContext.MyMessage for each message type.