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>

internal/form_builder.go

@@ -0,0 +1,49 @@
+package openai
+
+import (
+	"io"
+	"mime/multipart"
+	"os"
+)
+
+type FormBuilder interface {
+	CreateFormFile(fieldname string, file *os.File) error
+	WriteField(fieldname, value string) error
+	Close() error
+	FormDataContentType() string
+}
+
+type DefaultFormBuilder struct {
+	writer *multipart.Writer
+}
+
+func NewFormBuilder(body io.Writer) *DefaultFormBuilder {
+	return &DefaultFormBuilder{
+		writer: multipart.NewWriter(body),
+	}
+}
+
+func (fb *DefaultFormBuilder) CreateFormFile(fieldname string, file *os.File) error {
+	fieldWriter, err := fb.writer.CreateFormFile(fieldname, file.Name())
+	if err != nil {
+		return err
+	}
+
+	_, err = io.Copy(fieldWriter, file)
+	if err != nil {
+		return err
+	}
+	return nil
+}
+
+func (fb *DefaultFormBuilder) WriteField(fieldname, value string) error {
+	return fb.writer.WriteField(fieldname, value)
+}
+
+func (fb *DefaultFormBuilder) Close() error {
+	return fb.writer.Close()
+}
+
+func (fb *DefaultFormBuilder) FormDataContentType() string {
+	return fb.writer.FormDataContentType()
+}