move request_builder into internal pkg (#304) (#329)

* move request_builder into internal pkg (#304)

* add some test for internal.RequestBuilder

* add a test for openai.GetEngine

client.go

@@ -15,7 +15,7 @@ import (
 type Client struct {
 	config ClientConfig
 
-	requestBuilder    requestBuilder
+	requestBuilder    utils.RequestBuilder
 	createFormBuilder func(io.Writer) utils.FormBuilder
 }
 
@@ -29,7 +29,7 @@ func NewClient(authToken string) *Client {
 func NewClientWithConfig(config ClientConfig) *Client {
 	return &Client{
 		config:         config,
-		requestBuilder: newRequestBuilder(),
+		requestBuilder: utils.NewRequestBuilder(),
 		createFormBuilder: func(body io.Writer) utils.FormBuilder {
 			return utils.NewFormBuilder(body)
 		},
@@ -135,7 +135,7 @@ func (c *Client) newStreamRequest(
 	urlSuffix string,
 	body any,
 	model string) (*http.Request, error) {
-	req, err := c.requestBuilder.build(ctx, method, c.fullURL(urlSuffix, model), body)
+	req, err := c.requestBuilder.Build(ctx, method, c.fullURL(urlSuffix, model), body)
 	if err != nil {
 		return nil, err
 	}