`Newtonsoft.Json` to `System.Text.Json`

[erwinkramer]

Newtonsoft.Json should be replaced with System.Text.Json eventually.

[lucaslorentz]

Yeah, I wanted this for quite some time, it is a bit intricately connected to DTF though.
But should be doable.

[erwinkramer]

Maybe wait until DTF is migrated first, or offers flexible serialization support, see Azure/durabletask#1187

Don't know how complex this gets if you don't wait for that. Rather put the effort at the Azure/durabletask library first.