Support for custom scalars

[jomag]

It would be nice if graphql-flutter supported custom scalars. Like many others, I have a need for a date+time in my GraphQL types, and it would be great if I could add a custom scalar so that conversion between ISO8601 strings and DateTime objects was handled automatically.

[micimize]

This is a subset of the type generation problem: #26

For now, we leave that to other libraries:

• https://pub.dev/packages/artemis
• https://pub.dev/packages/gql_build
• https://github.com/micimize/major (my lib – janky slow and maximalist)

Ferry also has built-in codegen: https://pub.dev/packages/ferry