buildClientSchema implementation

The reference implementation of GraphQL has a utility function, [`buildClientSchema`](https://graphql.org/graphql-js/utilities/#buildclientschema), that is missing from this implementation.

The function converts the results of an introspection query to a `GraphQLSchema` instance.

`buildClientSchema` is key for schema federation (#492) and stitching (#329, #435), demonstrated by the [gatsby-source-graphql](https://www.gatsbyjs.org/packages/gatsby-source-graphql/) package.

Including this function would allow downstream packages to add federation and stitching features, and the creation of GraphQL analysis tools written in Go.

While federation and stitching might be out of scope for this project, the `buildClientSchema` building block seems within scope, being part of the reference implementation.

Thanks for the great work on this package!