Add the infrastructure for shim signing

[sayanchowdhury]

TODO before merge

• [ ] Bump shim pacakge revision

• [ ] Changelog entries added in the respective changelog/ directory (user-facing change, bug fix, security fix, update)

• [ ] Inspected CI output for image differences: /boot and /usr size, packages, list files for any missing binaries, kernel modules, config files, kernel modules, etc.