Add a crate for configuring and using Nitrokey's NetHSM

This adds integration for Nitrokey's NetHSM.

The crate is currently fully integration tested using upstream's nethsm container.

The code can probably still see some cleanups but seems to work as intended, apart from two known issues: https://github.com/Nitrokey/nethsm/issues/5

added scopefeature label

requested review from @svenstaro and @anthraxx

It is unclear to me which parts are generated and which are hand-written (so I don't know what to review). Also, there appear to be no instructions on how to re-generate the sources using OpenAPI Generator and no Just target for it either. I would prefer if that were added so people know how to reproduce this.

Wouldn't it make sense to release the nethsm crate independently on crates.io? After all, it doesn't really belong into this repo. I'd like to keep the code focused on the problem at hand and not ideally have a large dependency here that other may otherwise also benefit from for their own projects.