nettle: support deriving ML-DSA public key from expanded secret key

nettle: support deriving ML-DSA public key from expanded secret key

RFC 9881 defines 3 private key formats for ML-DSA: "seed",
"expandedKey" and both. When it is "expandedKey", a non-trivial
conversion is required to derive a public key, which is now
implemented in leancrypto through lc_dilithium_pk_from_sk. This patch
modifies the pk_fixup backend function to use it to derive a public
key when importing a private key.

Signed-off-by: Daiki Ueno <ueno@gnu.org>