Dev: github action for check GPG KEY

Shield does not accept commits that are not signed with GPG KEY, so there is an Github Action to automatically check the signature of commits and add a tag **GPG-Signing needed** and a comment to explain:

How to **[setup a GPG key](https://github.com/codeigniter4/CodeIgniter4/blob/develop/contributing/signing.md#secure-signing)**
How to **[GPG-Signing Old Commits](https://github.com/codeigniter4/CodeIgniter4/blob/develop/contributing/workflow.md#gpg-signing-old-commits)**

首先阅读链接的 signing.md 和 workflow.md 指南，以了解仓库预期的 GPG-signing workflow。将 action 的完成标准定义为检测未签名的提交、应用 GPG-Signing needed 标签，以及发布一条包含相关设置链接的说明性评论。

由索引模型根据 Issue 内容生成。