Add stub implementation of SlhDsaCng and algorithm identifiers

[vcsjones]

This adds an Experimental implementation of SLH-DSA backed by CNG that, for now, only throws PNSE. This will ensure we get the API surface shipped so that the implementation can be filled in later.

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[Copilot]

Pull Request Overview

This PR adds a stub implementation of the Stateless Hash-based Digital Signature Algorithm (SLH-DSA) backed by CNG to establish the API surface for future implementation. The changes introduce experimental SLH-DSA algorithm identifiers and a CNG-based class that currently throws PlatformNotSupportedException for all operations.

• Adds SLH-DSA algorithm identifiers to CNG classes with experimental attributes
• Creates SlhDsaCng class with complete API surface but stub implementations
• Updates project files to include the new SlhDsaCng implementation

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 7 comments.

Show a summary per file

File	Description
CngAlgorithmGroup.cs	Adds SlhDsa algorithm group property with experimental attribute
CngAlgorithm.cs	Adds SlhDsa algorithm property with experimental attribute
System.Security.Cryptography.csproj	Includes SlhDsaCng.cs in compilation
System.Security.Cryptography.cs	Adds SlhDsaCng class to reference assembly
Microsoft.Bcl.Cryptography.csproj	Includes SlhDsaCng.cs in BCL project
SlhDsaCng.cs	Implements stub SlhDsaCng class with PNSE-throwing methods

[PranavSenthilnathan]

/ba-g known timeout issues and wasm failures