Move form_builder into internal pkg. (#311)

* Move form_uilder into internal pkg.

* Fix import of audio.go

* Reorganize.

* Fix import.

* Fix

---------

Co-authored-by: JoyShi <joy.shi@sap.com>

client.go

@@ -7,6 +7,8 @@ import (
 	"io"
 	"net/http"
 	"strings"
+
+	utils "github.com/sashabaranov/go-openai/internal"
 )
 
 // Client is OpenAI GPT-3 API client.
@@ -14,7 +16,7 @@ type Client struct {
 	config ClientConfig
 
 	requestBuilder    requestBuilder
-	createFormBuilder func(io.Writer) formBuilder
+	createFormBuilder func(io.Writer) utils.FormBuilder
 }
 
 // NewClient creates new OpenAI API client.
@@ -28,8 +30,8 @@ func NewClientWithConfig(config ClientConfig) *Client {
 	return &Client{
 		config:         config,
 		requestBuilder: newRequestBuilder(),
-		createFormBuilder: func(body io.Writer) formBuilder {
-			return newFormBuilder(body)
+		createFormBuilder: func(body io.Writer) utils.FormBuilder {
+			return utils.NewFormBuilder(body)
 		},
 	}
 }