Parse metadata of hs_ed25519_secret_key

We currently skip the first 32 bytes of the hs_ed25519_secret_key when parsing in pkg/okey. It would be nice to check that this initial metadata is not malformed.

Maybe a good place to start looking for inspiration: https://gitlab.torproject.org/tpo/core/tor/-/blob/main/src/feature/keymgt/loadkey.c#L379