Add stubs for CompositeMLDsa APIs

[PranavSenthilnathan]

Adds new approved APIs as stubs that throw PNSE. We anticipate requests to use Composite ML-DSA before .NET 11, so we will put the APIs in .NET 10 and implement them in a servicing update if/when necessary.

Contributes to #118320

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[Copilot]

Pull Request Overview

This PR adds stub implementations for Composite ML-DSA APIs in .NET 10 to enable early access before full implementation. The APIs are marked experimental and throw PlatformNotSupportedException to indicate they're not yet implemented.

Key changes:

• Adds new CompositeMLDsaCng class with CNG-based implementation stubs
• Extends X.509 certificate functionality with Composite ML-DSA support
• Adds Composite ML-DSA support to CMS signing and certificate request operations

Reviewed Changes

Copilot reviewed 14 out of 14 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
X509SignatureGenerator.cs	Adds CreateForCompositeMLDsa method stub
X509Certificate2.cs	Adds methods for getting/setting Composite ML-DSA keys
PublicKey.cs	Adds constructor for Composite ML-DSA keys
CertificateRequest.cs	Adds constructors accepting Composite ML-DSA keys
Cng.NotSupported.cs	Adds CompositeMLDsaCng stub for non-CNG platforms
System.Security.Cryptography.csproj	Includes new CNG implementation files
System.Security.Cryptography.cs	Adds reference assembly definitions
CmsSigner.cs	Adds constructor accepting Composite ML-DSA private key
System.Security.Cryptography.Pkcs.netcoreapp.cs	Adds CMS signer reference
X509CertificateKeyAccessors.cs	Adds extension methods for Composite ML-DSA
Microsoft.Bcl.Cryptography.csproj	Includes CNG files in BCL project
Microsoft.Bcl.Cryptography.Forwards.cs	Adds type forwarding for CompositeMLDsaCng
CompositeMLDsaCng.cs	New CNG implementation class definition
CompositeMLDsaCng.Windows.cs	Windows-specific CNG implementation stubs

[PranavSenthilnathan]

/cc @jeffhandley @artl93 Adding new PQC APIs for RC1