feat: Add `nethsm-backup` library and binary

Cargo.lock

Cargo.toml

nethsm-backup/Cargo.toml

nethsm-backup/README.md

nethsm-backup/src/bin/main.rs

nethsm-backup/src/lib.rs

nethsm-backup/tests/backup-decrypt.rs

nethsm-cli/Cargo.toml

nethsm-cli/README.md

nethsm-cli/src/cli/system.rs

nethsm-cli/src/main.rs

nethsm-config/src/prompt.rs

nethsm/Cargo.toml

nethsm/src/lib.rs

nethsm/src/user.rs