Add listen API support

[rdyv]

We have listener API which is public but is not supported by Go client. This PR adds support for it.

This change was tested locally with this Go program:

package main

import (
	"context"
	"log"

	sanity "github.com/sanity-io/client-go"
)

func main() {
	client, err := sanity.VersionV20210325.NewClient("myProjectId", sanity.DefaultDataset,
		sanity.WithCallbacks(sanity.Callbacks{
			OnQueryResult: func(result *sanity.QueryResult) {
				log.Printf("Sanity queried in %d ms!", result.Time.Milliseconds())
			},
		}))
	if err != nil {
		log.Fatal(err)
	}

	ctx := context.Background()
	result, err := client.Listen("*").Do(ctx)
	if err != nil {
		log.Fatal(err)
	}

	for {
		event, err := result.Next()
		if err != nil {
			log.Fatal(err)
		}
		log.Printf("Event =>")
		log.Printf("\tType: %s", event.Type)
		log.Printf("\tID: %s", event.ID)
		log.Printf("\tData: %s\n\n", string(*event.Data))
	}
}