Add license headers and related checks

The LICENSE file is missed currently.

Before moving the C++ client from Pulsar main repo here, the license check was performed by `mvn license:check`. After adding the LICENSE file, we should add a license verification workflow.

首先檢查儲存庫，尋找缺少的 LICENSE 檔案、現有的授權標頭以及 workflow 設定。檢視先前的 `mvn license:check` 檢查如何套用於 C++ 用戶端，並決定對等的驗證範圍。當 LICENSE 檔案和必要的標頭都已存在，且有自動化 workflow 對其進行驗證時，即表示完成。

由索引模型根據 Issue 內容生成。