Support SLH-DSA with Windows NCrypt

[bartonjs]

When Windows adds support for SLH-DSA to NCrypt (https://techcommunity.microsoft.com/blog/microsoft-security-blog/post-quantum-cryptography-comes-to-windows-insiders-and-linux/4413803), we need to take advantage of it. In addition to "plumbing" work, we need to expose it via SlhDsaCng, and associated API updates (e.g. CngAlgorithm).

[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.