ML-DSA support on CryptoKit

[vcsjones]

Apple introduced ML-DSA in CryptoKit in Apple Platform 26. We should light up MLDsa on Apple platforms with CryptoKit.

Some notable details on CryptoKit.

1. No Security Category < 3, so that means ML-DSA-44 parameter set is not supported.

2. CryptoKit does not support FIPS-204 private keys. Only private seeds. Practically, that means ExportMLDsaPrivateKey, ExportMLDsaPrivateKeyPem, and their import variants are going to throw PlatformNotSupportedException.

3. We need to decide how PKCS#8 imports work. ML-DSA private keys, as of IETF draft 13, are seed, expandedKey, and both.

   1. seed will work just work.
   2. expandedKey is going to throw.
   3. both, we can do one of two ways. The challenge here is what to do with expandedKey in the SEQUENCE. We have no ability to validate the expandedKey. We can ignore it (inconsistent with other platforms, as well as allowing inconsistencies in private key structures) and just import the seed, or we can always throw because don't have the information needed to validate the expandedKey. Throwing is tempted, except for the fact that by default OpenSSL CLI produces both if the key has a seed. So OpenSSL CLI will create PKCS#8 private keys that are both if it can. This is different from .NET on OpenSSL however, where we will always produce seed if we can, otherwise expandedKey, but never both.

4. No X.509 or key services support. Apple does not support certificates with ML-DSA signatures or keys. So this support from CryptoKit only offers MLDsa primitive support. That means X509Certificate2 APIs that work with keys will not be supported. SubjectPublicKeyInfo on X509Certificate2 and in PublicKey are going to work.

5. No external Mu support.

6. No PreHash support.

[dotnet-policy-service]

Tagging subscribers to 'os-ios': @vitek-karas, @kotlarmilos, @steveisok, @akoeplinger
See info in area-owners.md if you want to be subscribed.

[dotnet-policy-service]

Tagging subscribers to 'os-tvos': @vitek-karas, @kotlarmilos, @steveisok, @akoeplinger
See info in area-owners.md if you want to be subscribed.

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-security, @bartonjs, @vcsjones
See info in area-owners.md if you want to be subscribed.

[vcsjones]

I will do SHA-3 support first for CryptoKit in ##116511. SHA-3 support is needed for some of the test vectors of SignedCms and others consumers of ML-DSA.