This PR adds two commands to the `justfile`:
  - `just generate shell_completions nethsm-cli`
  - `just generate manpages nethsm-cli`

It also makes the `nethsm-cli` expose a minimal set of APIs for
exposing CLI interface for `clap_allgen`.

Fixes: #45


Signed-off-by: Wiktor Kwapisiewicz <wiktor@metacode.biz>