Add eid-mw: authenticate & sign with Belgian eID identity cards

Adds limited but useful support for Belgian electronic ID cards.

Limited for now, because:
- While a Firefox extension is included and installed, no effort is made to integrate it. I use Chromium.
- The library should work with all NSS-compatible browsers, but must be registered manually by and for each user. Instructions are included.
Since this package installs the libfile, you should replace this with the full path to the package. See e.g. https://github.com/NixOS/nixpkgs/blob/310acaf1a05b80cc42399eb30e66674c74661b82/pkgs/development/libraries/haskell/hoogle/hoogle-local-wrapper.sh and its .nix file for an example.