feat: introduce BindResponseHeaders option

[hgiasac]

close #161

Introduce a new BindResponseHeaders option to retrieve response headers via binding.

headers := http.Header{}
err := client.Query(context.TODO(), &q, nil, graphql.BindResponseHeaders(&headers))
if err != nil {
  panic(err)
}

fmt.Println(headers.Get("content-type"))
// application/json

[github-actions]

Package	Line Rate	Health
github.com/hasura/go-graphql-client	68%	➖
github.com/hasura/go-graphql-client/ident	100%	✔
github.com/hasura/go-graphql-client/pkg/jsonutil	84%	✔
Summary	73% (1433 / 1973)	➖

Minimum allowed line rate is 60%