HTTP Client as an interface

[josipsumeckispread]

Having HTTP Client as an interface allows us to use more complex http client implementations. The one I am personally interested at is github.com/sethgrid/pester which allows us to use resilient requests to remote GraphQL server by implementing a retry strategy on underlying HTTP requests.
The rest of the repository requires no change. All the existing generated code will be compatible with the proposed change.

Example:

package ein

import (
	"context"
	"net/http"
	"time"

	"github.com/sethgrid/pester"

        "github.com/Yamashou/gqlgenc/clientv2"
)

func NewEinHttpClientWithRetry(roundTripper http.RoundTripper) clientv2.HttpClient {
	client := pester.New()
	client.Transport = roundTripper
	client.Timeout = 120 * time.Second
	client.Concurrency = 1
	client.MaxRetries = 5
	client.Backoff = pester.ExponentialJitterBackoff
	client.RetryOnHTTP429 = true
	return client
}