Feature Request: set type QueryOptions and MutationOptions like Apollo

[isaiahtaylorhh]

Is your feature request related to a problem? Please describe.
In Apollo, you can specify mutate(MutationOptions<CreateTestInput>{variables: {...}}). This is very powerful because it enforces the schema's requirements for a query or mutation. This is especially powerful when used in conjunction with automatic type generation like this: https://github.com/micimize/graphql-to-dart .

Describe the solution you'd like

class MutationOptions<T> extends BaseOptions {
  MutationOptions({
    // ...
    Map<String, T> variables, // used to be <String, dynamic>
    // ...
  }) : super(
    // ...
  );
}

Same for query.

Additional context
I've got this running locally and it works great. If I get some extra time I'll put it up in a PR but that is unlikely.

Usage:

await client.mutate(
  MutationOptions<CreateTestInput>(
    documentNode: gql(myMutation),
    variables: {
      "input": myTestInput, // myTestInput is CreateTestInput
    })
);

[isaiahtaylorhh]

This could also be extended to the QueryResult so that QueryResult.data is MySchemaType.

[micimize]

We wouldn't be able to do Map<String, T>, but we can use the generated variable type as well.

[micimize]

Merging in to #636 and continuing convo there