[REQ] [csharp-netcore] Support the System.Text.Json source generator.

Created by: teo-tsirpanis

Is your feature request related to a problem? Please describe.

The modes of the csharp-netcore generator that support System.Text.Json use the JsonSerializer methods that perform reflection, making the generated client code incompatible with trimming and AOT.

Describe the solution you'd like

The generator should emit a partial class deriving from JsonSerializerContext and decorated with [JsonSerializable] attributes, that will allow the System.Text.Json source generator to emit the serialization logic at compile time, and then pass this context to a suitable JsonSerializer method. We might need an opt-in or opt-out switch, in case there are any incompatibilities.

Describe alternatives you've considered

There are no supported alternatives.