returns NetworkError when the request response returns a non-200 http status code

[LukeSparkLayer]

This PR adds a new error type: NetworkError

This error contains the status code and response body which were previously returned as the error message.

This change has two benefits:

• Allows systems that depend on this package to be able to check for error response codes by using go's inbuilt errors package and matching on the error code, rather than string matching on the error message.
• Returns the body of the request as an io.Reader to reduce unnecessary data being passed around.

// Before 
err := client.Query(ctx, query, variables)
if err != nil {
	serviceErrRegex := regexp.MustCompile(`^non-200 OK status code: 5\d\d.+`)
	if serviceErrRegex.MatchString(err.Error()) {
		// handle 5xx status response
	}
}

// After
err := client.Query(ctx, query, variables)
statusErr := graphql.NetworkError{}
if errors.As(err, &statusErr) && statusErr.StatusCode() >= 500 && statusErr.StatusCode() < 600 {
	// handle 5xx status response
}

[LukeSparkLayer]

Thanks for your quick reviews @hgiasac. I have added the changes suggested. Let me know if there's any others needed.

[hgiasac]

@LukeSparkLayer the change looks good. However, you need to fix the error message in unit tests as well. We can merge after all tests are passed

[hgiasac]

LGTM. Thanks for contributing