Add testable request builder (#162)

* Add testable request builder

* improve tests

request_builder.go

@@ -0,0 +1,40 @@
+package openai
+
+import (
+	"bytes"
+	"context"
+	"net/http"
+)
+
+type requestBuilder interface {
+	build(ctx context.Context, method, url string, request any) (*http.Request, error)
+}
+
+type httpRequestBuilder struct {
+	marshaller marshaller
+}
+
+func newRequestBuilder() *httpRequestBuilder {
+	return &httpRequestBuilder{
+		marshaller: &jsonMarshaller{},
+	}
+}
+
+func (b *httpRequestBuilder) build(ctx context.Context, method, url string, request any) (*http.Request, error) {
+	if request == nil {
+		return http.NewRequestWithContext(ctx, method, url, nil)
+	}
+
+	var reqBytes []byte
+	reqBytes, err := b.marshaller.marshal(request)
+	if err != nil {
+		return nil, err
+	}
+
+	return http.NewRequestWithContext(
+		ctx,
+		method,
+		url,
+		bytes.NewBuffer(reqBytes),
+	)
+}