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)**

Start by reading the linked signing.md and workflow.md guidance to understand the repository’s expected GPG-signing workflow. Define the action’s completion criteria as detecting unsigned commits, applying the GPG-Signing needed tag, and posting an explanatory comment with the relevant setup links.

Written by the indexing model from the issue text.