Add Cosign (Sigstore) Tool?

[doudou0720]

Would you please add Cosign?

https://docs.sigstore.dev/cosign/system_config/installation/#with-the-cosign-binary-or-rpmdpkg-package

[bastianeicher]

Template and watch script are merged. Running them now to generate the <implementation>s. Will update ticket once those are ready.

[bastianeicher]

The feed is now ready to use: https://apps.0install.net/utils/cosign.xml