Define (or reuse) configuration module for standalone signing-only executable

With #34 we want to create an executable, that is fully non-interactive and used for signing operations only. It is meant to be called over SSH and rely on a dedicated configuration.

The configuration needs to be able to match users on the called system (in particular the one calling the executable) to operator users on the nethsm and allow for configuration of the TLS verification method used for interaction with the NetHSM API.

In the nethsm-cli crate we have a config module that can, if feasible, be moved to its own crate and be reused for this purpose, as it covers parts of this task already.