feat: decode extensions from response via BindExtensions option

[hgiasac]

The response map may also contain an entry with key extensions. To decode this field you need to bind a struct or map pointer. The client will optionally unmarshal the field using json decoder.

var q struct {
	User struct {
		ID   string `graphql:"id"`
		Name string `graphql:"name"`
	}
}

var ext struct {
	ID     int    `json:"id"`
	Domain string `json:"domain"`
}

err := client.Query(context.Background(), &q, map[string]interface{}{}, graphql.BindExtensions(&ext))
if err != nil {
	t.Fatal(err)
}

[github-actions]

Package	Line Rate	Health
github.com/hasura/go-graphql-client	67%	➖
github.com/hasura/go-graphql-client/ident	100%	✔
github.com/hasura/go-graphql-client/pkg/jsonutil	84%	✔
Summary	72% (1412 / 1959)	➖

Minimum allowed line rate is 60%