introspection: create a graphql schema from introspection query with GraphQLClientSchema::new

[lrlna]

Description

Given a result from an Introspection Query, we should be able to construct a GraphQLClientSchema.

Detailed design

GraphQLClientSchema::new should be an implementation of what buildClientSchema does in graphql-js. That is, given an introspection result:

1. Traverse all available types in result's __schema and get their type definitions

2. Include only Scalar and Introspection types that are present in the query result

3. Include root Query, Mutation, and Subscription types

4. Include introspection supported Directives

5. Construct a Schema object with the following definitions:

   pub struct GraphQLClientSchema {
       description: String,
       query: QueryType,
       mutation: MutationType,
       subscription: SubscriptionType,
       types: SchemaTypes,
       directives: Directives,
   }

Test plan

To ensure our generated SDL is inline with javascript's reference implementation, we need to implement all of the test cases in listed in graphql-js/buildClientSchema.

References

• graphql-parser: get an AST of a schema to construct a correct.
• graphql-client-codegen: an existing Schema type.
• introspection spec: spec document outlining possible introspection types.

This issue is part of introspection work discussed in this internal scope document.