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)**