feat: Add `signstar-sign`

Cargo.lock

Cargo.toml

REUSE.toml

justfile

nethsm-config/src/config.rs

nethsm/src/openpgp.rs

signstar-config/tests/non_admin_credentials/mod.rs

signstar-sign/Cargo.toml

signstar-sign/README.md

signstar-sign/src/main.rs

signstar-sign/tests/fixtures/cert.not-pem

signstar-sign/tests/fixtures/key.not-pem

signstar-sign/tests/fixtures/signstar-config-plaintext.toml

signstar-sign/tests/integration.rs

signstar-sign/tests/operator_signing/mod.rs

signstar-sign/tests/utils/mod.rs