Simplify graphql.Client interface (MakeRequest)

[benjaminjkraft]

The graphql.Client interface is scarily complex, for something we have to support forever. We may want to try to make it a bit more constrained, with specific hooks for specific things, so that callers can't do weird things. But we'll need to see how people actually use it.

At Khan, we currently:

• munge the URL to add the opname
• munge the error (if any), wrapping, and potentially adding some of the variables

Other potential use cases:

• caching (so Caching #29 can be done in third-party code)
• send a hash (likewise Send a hash instead of the query #20) or minify (Minify queries #7)
• non-HTTP transports
• non-JSON transports (this will be fairly hard since we'll have to generate unmarshalers for them; I'm not sure if anyone even does this though)

[benjaminjkraft]

Another option is to do

type Request struct {
  OperationName string
  Query string
  Variables map[string]interface{}
}
type Client interface {
  MakeRequest(ctx context.Context, request *Request, retval interface{})
}

and then we could at least add fields.

We could even replace retval interface{} with returning the json.RawMessage, or with passing you unmarshal(string) interface{} and having you return the interface{}, although both are a bit ugly and I'm not sure either is quite flexible enough.

[benjaminjkraft]

A great solution similar to my above comment fell out in #184, so I think we are all set here.