refactor: renew API client (request & response abstraction by struct)

[ninoseki]

A bit (or very) opinionated PR to refactor the API module.

I think having structs for handling request & response is better since it's more open to extend.

For example,

Before

func (cli *Client) parseResponse(resp *http.Response) (*JSONResponse, error) {
	jsonResp := &JSONResponse{}

	if !strings.HasPrefix(resp.Header.Get("Content-Type"), "application/json") {
		return nil, fmt.Errorf("expecting JSON response from %s %s",
			resp.Request.Method, resp.Request.URL.String())
	}

	var reader = resp.Body
	read, err := io.ReadAll(reader)
	if err != nil {
		return nil, err
	}
	// consider 2xx response as successful
	if resp.StatusCode >= 200 && resp.StatusCode < 300 {
		jsonResp.Raw = json.RawMessage(read)
		return jsonResp, nil
	}

	jsonErr := &JSONError{}
	err = json.Unmarshal(read, jsonErr)
	if err != nil {
		return nil, err
	}
	return nil, jsonErr
}

...

func (cli *Client) DoWithJSONParse(req *http.Request) (*JSONResponse, error) {
	resp, err := cli.httpClient.Do(req)
	if err != nil {
		return nil, err
	}
	defer resp.Body.Close() //nolint:errcheck

	return cli.parseResponse(resp)
}

(It needs a parsing method (func parse...) & customized Do method (func DoWith...) per format)

After

func (r *Response) ToJSON() (*json.RawMessage, error) {
	body, err := r.ToBytes()
	if err != nil {
		return nil, err
	}

	contentType := r.GetContentType()
	if !strings.Contains(contentType, "application/json") {
		return nil, fmt.Errorf("response is not JSON, content-type: %s", contentType)
	}
	raw := json.RawMessage(body)
	return &raw, nil
}

...

resp, err := client.NewRequest().Get("/")
if err != nil {
	return err
}
resp.ToJSON()

(It only needs a response's method per format)

I think the later interface is more easy to add a support of a new format, etc.

[cdnsyseng]

@ninoseki looks fine, please look at #69 before merging this