Support for custom scalars

[wereHamster]

I'd like to store objects in the cache that should be treated as scalars by apollo. When writing the objects to the cache, no __typename should be required, and while reading the object no subfields should be required.

As an example, let's assume that I have such a custom TypeScript type:

type Highlight = { foo: string } | { bar: number } | { baz: boolean }

and would like to initialize the apollo client as follows:

const client = new ApolloClient({
  clientState: {
    defaults: {
      highlight: { foo: "this" }
    },
    resolvers: {
      Mutation: {
        updateHighlight: (_, { newValue }, { cache }) => {
          cache.writeData({ data: { highlight: newValue }});
          return null;
        }
      }
    }
  }
})

So that I can use it in queries, without needing to specify the fields:

query {
  highlight
}

In a schema I'd specify Highlight as a custom scalar, but I don't see any examples how to do that in apollo-link-state.