scd:nks: Add support for signing plain SHA-2 digests.

* scd/app-nks.c (do_sign): Handle plain SHA-2 digests and verify
encoding of ASN.1 encoded hashes.

This makes it possible to create CSRs for NetKey card keys which are
signed with SHA256 by default.

(cherry picked from commit 8fe976d5b9a0f2902868737dd502c749565222a6)

• GnuPG-bug-id: T5184