modsign: Enable ML-DSA module signing

modsign: Enable ML-DSA module signing

Allow ML-DSA module signing to be enabled.

Note that OpenSSL's CMS_*() function suite does not, as of OpenSSL-3.6,
support the use of CMS_NOATTR with ML-DSA, so the prohibition against using
signedAttrs with module signing has to be removed.  The selected digest
then applies only to the algorithm used to calculate the digest stored in
the messageDigest attribute.  The OpenSSL development branch has patches
applied that fix this[1], but it appears that that will only be available
in OpenSSL-4.

[1] https://github.com/openssl/openssl/pull/28923

sign-file won't set CMS_NOATTR if openssl is earlier than v4, resulting in
the use of signed attributes.

The ML-DSA algorithm takes the raw data to be signed without regard to what
digest algorithm is specified in the CMS message.  The CMS specified digest
algorithm is ignored unless signedAttrs are used; in such a case, only
SHA512 is permitted.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Jarkko Sakkinen <jarkko@kernel.org>
cc: Eric Biggers <ebiggers@kernel.org>
cc: Lukas Wunner <lukas@wunner.de>
cc: Ignat Korchagin <ignat@cloudflare.com>
cc: Stephan Mueller <smueller@chronox.de>
cc: Herbert Xu <herbert@gondor.apana.org.au>
cc: keyrings@vger.kernel.org
cc: linux-crypto@vger.kernel.org