feat: introduce dynamic GraphQL Builder

[hgiasac]

Introduce the Builder structure that wraps the naive [][2]interface{} type. It helps construct multiple queries to a request that needs to be conditionally added.

You might need to dynamically multiple queries or mutations in a single request. It isn't convenient with static structures. Builder helps us construct many queries flexibly.

For example, to make the following GraphQL mutation:

query($userId: String!, $disabled: Boolean!, $limit: Int!) {
	userByPk(userId: $userId) { id name }
	groups(disabled: $disabled) { id user_permissions }
	topUsers: users(limit: $limit) { id name }
}

# variables {
# 	"userId": "1",
# 	"disabled": false,
# 	"limit": 5
# }

You can define:

type User struct {
	ID string
	Name string
}

var groups []struct {
	ID string
	Permissions []string `graphql:"user_permissions"`
}

var userByPk User
var topUsers []User

builder := graphql.NewBuilder().
	Query("userByPk(userId: $userId)", &userByPk).
	Query("groups(disabled: $disabled)", &groups).
	Query("topUsers: users(limit: $limit)", &topUsers).
	Variables(map[string]interface{}{
		"userId": 1,
		"disabled": false,
		"limit": 5,
	})

query, variables, err := builder.Build()
if err != nil {
	return err
}

err = client.Query(context.Background(), query, variables)
if err != nil {
	return err
}

// or use Query / Mutate shortcut methods
err = builder.Query(client)
if err != nil {
	return err
}

[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	86%	✔
Summary	73% (1539 / 2114)	➖

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