T7579: added workflow for CLA

[lemeshovich]

How to test

1. Create a new pull request from a GitHub user who has never signed the CLA before.
2. Upon PR creation or update, the GitHub Action will automatically run and check for a valid CLA signature.
3. If the user has not signed the CLA:
   3.1 A comment will appear on the pull request prompting them to sign it.
   3.2 The user must respond with the exact phrase: I have read the CLA Document and I hereby sign the CLA
4. The signature will be stored in the central repository vyos/vyos-cla-signatures under signatures/version1/cla.json.
5. After the signature is added, the CLA check will pass and the comment will be updated accordingly.

The action will ignore bots in the allowlist (e.g. vyosbot and others) and will not require them to sign the CLA.

Checklist:

• My code follows the process laid out in the documentation
• I have updated or added any unit tests accordingly
• I have updated or added any documentation accordingly