Add cosign-based signing, attestation, and verification for modelkits

[cr3ativ3cod3r]

Description

This PR adds support for signing, attesting, and verifying modelkits using the kit commands directly. These commands use cosign internally. The users doesn't have to switch between multiple tools. The verify command enables the user to run both verify and verify attestation using a single command.

Linked issues

closes #857

AI-Assisted Code

• [ ] This PR contains AI-generated code that I have reviewed and tested
• [x] I take full responsibility for all code in this PR, regardless of how it was created