ML-KEM: CryptoKit support

[vcsjones]

Apple introduced ML-KEM support to CryptoKit that will be available on Apple Platform 26+. We should allow MLKem to use CryptoKit on Apple platforms.

Because CryptoKit is an opinionated framework, it has a couple of omissions.

1. No security category 1. That means no ML-KEM-512, only ML-KEM-768/1024.
2. Seeds only for private keys. That means Decapsulation key import / export cannot be implemented, and PKCS#8 files will only support the seed CHOICE.
   • We could support both, but that would effectively mean ignoring the expandedKey, which does not feel like the right thing to do.
3. CryptoKit is primitives only, so this would mean no PKCS#12 and certificate support. macOS may support them, but it won't be in CryptoKit. If that support becomes available it will be tracked in a separate issue.

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[vcsjones]

This will have to be post 10.0.0 due to the required Apple SDK not being available by the time 10.0.0 RTMs.