Add SourceGenerator support to the standalone tool

[attilah]

Is your feature request related to a problem? Please describe.
To enable the generated client to have the serializers source generated, the tool could generate an <ApiTypeName>JsonSerializerContext class which inherits from JsonSerializerContext and add all the type names with JsonSerializable to the class, so the serializer source generator could kick in and do its job.

This feature can only be added to the standalone tool because source generators cannot interact with each other currently.

Describe the solution you'd like
An <ApiTypeName>JsonSerializerContext class would be generated as well.

Describe alternatives you've considered
Manually write this class by hand which is tedious.

Additional context
More info on it: https://devblogs.microsoft.com/dotnet/try-the-new-system-text-json-source-generator/

[christianhelle]

@attilah thanks for taking the time to suggest this

I'm unsure how the Refit generated code works with System.Text.Json source generators but I can investigate it. I'll see what I can do.

[christianhelle]

@all-contributors please add @attilah for ideas

[allcontributors]

@christianhelle

I've put up a pull request to add @attilah! 🎉