Extract optional functionality into go plugins

[anatol]

There is some amount of functionality that depends on external tools/libraries. Like FIDO2 support. Currently it uses command-line tools for communication. But this interface is usually brittle and inconvenient. It would be great to use libraries with C API directly. But instead of linking to these libs it would make sense to use go plugins to have these dependencies truly optional.

[c3Ls1US]

While brittle, inconvenient and in theory less performant, it's simpler and lets the init process be free of CGO. See #240