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.

首先检查仓库，查找缺失的 LICENSE 文件、现有的许可证头以及 workflow 配置。查看之前的 `mvn license:check` 检查如何应用于 C++ 客户端，并确定等效的验证范围。当 LICENSE 文件和所需的头文件均已存在，并且有一个自动化 workflow 对其进行验证时，即视为完成。

由索引模型根据 Issue 内容生成。