Split backend and extension

[robin-nitrokey]

This patch splits the trussed-auth crate into two crates: trussed-auth only defines the AuthExtension and can be used by clients or other backends implementing the extension. trussed-auth-backend contains the AuthBackend that implements the extension using the filesystem.

[sosthene-nitrokey]

I think we should be constistent with the other extensions/backends.

The implementation goes at the top level, the extension goes in to an extension top-level directory.

I fundamentally don't care much for it, but It should be consistent across crates.

[robin-nitrokey]

I think the main difference to the other repositories is that in this case, the extension is the main component, not the backend. So what should, for example, happen with the changelog? It would be consistent to have the backend changelog in . and the extension changelog in extension, but I would find it confusing if the root changelog exists but does not contain the changes for the main crate of the repository.