Create executable for OpenPGP signing

For basic OpenPGP signing operations we need to create a simple one-operation executable (e.g. signstar-sign), that allows us to create OpenPGP signatures for messages (and potentially further versions of OpenPGP, or other technologies such as SSH in the future).

The surrounding setup for this executable is explained in greater detail in !26 (merged)

This executable needs to be configurable by a well-defined configuration file (configuration file handling may be abstracted from config module in the nethsm-cli crate if needed), that allows to map the calling unix user to a user in the operator role on a set of NetHSM devices. The configuration file must allow configuring one or more NetHSM devices, which are considered to provide identical users and keys and act as failovers if one is not reachable. The configuration file format further must allow for configuring the desired signature output format (e.g. OpenPGPv4, OpenPGPv6 or SSH) and default to OpenPGPv4 (for now).